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So, let’s say 300-ish modern American Soldiers/Marines are sent back to the civil war. They lack any heavy weaponry, but overall this is what their armament consists of:

250 of them have basic M4 rifles and M9 pistols.

25 of them have M249 SAW’s.

And the last 25 have Barret .50 cal sniper rifles.

They have advanced weapons and tech, but there are only 300 of them, so I’d like to ask: What would be the best, most tactically logical role to place these men in.

Clarification: They are fighting on the Union side, and as for ammo, they have consistent resupply of it.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Oct 7, 2021 at 3:26
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    $\begingroup$ I think it would make a big difference in how this goes down whether your 300 also have the knowledge of the history between when they came from and when they arrived. Do they get sent to their own past? Do they get sent to an alternative timeline? Is it a closed "it already happened" time loop deal like that one Harry Potter book? Do they keep their memories or just the combat training / general knowledge? Are they prepared specifically for this mission or are these some abstract standard issue unnamed E1s? $\endgroup$ Oct 11, 2021 at 9:17
  • $\begingroup$ I've got a question about this, Why did they send back a random group of soldiers instead of an actual unit? & where are the people needed to actually sustain a group of soldiers? Militarie don't just deploy random groups of men, they deploy actual units. $\endgroup$
    – OT-64 SKOT
    Feb 11, 2022 at 7:20
  • $\begingroup$ A typical soldier has less than 500 rounds for their gun on them if they go into combat, and most likely less if they have not geared up for that. $\endgroup$
    – Trish
    May 5, 2022 at 6:09
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    $\begingroup$ I’m voting to close this question because it's almost impossible for the OP to provide enough detail for an answer to a question of this type. How are the units trained? What are their specialties? At what point in the war? To what regiment would they be assigned? How are we to account for obvious changes in the war's outcome? Entire books have been written about alternative history Civil War, so this Q violates a number of rules, including the High Concept Question policy. $\endgroup$
    – JBH
    May 5, 2022 at 17:54

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I feel like a lot of other answers focused on things other than the combat effectiveness that modern troops would have, which is what I feel like the question is getting at. Yes, their knowledge is invaluable, their training would be helpful, and their perfect knowledge of the future is a huge advantage. But I think the OP was focused on how they could be used in direct combat. That being said, here is my take.

Campaign Strategy


First, large-scale strategy. You would want to save the use of these troops for a time when the enemy forces are extended. Once they are used the enemy would change tactics, and while you would still have an advantage, the massive advantage of using them in surprise would be gone. These armies would adapt, just as troops in WW1, WW2, and modern conflicts changed tactics based on new technologies. They would be technologically out-classed, but they aren't stupid.

I would attempt to goad the enemy into an attack with their main forces. First, I would engage a superior enemy force and then start a strategic retreat into a defensive position where the modern troops could be deployed. I would then use the modern forces to decimating effect in that battle and continue to harry them until the entire force surrenders. That alone could end the war if planned well enough, especially if they see the effectiveness of your new weapons.

Also, artillery at the time was incredibly effective. Concentrating your modern troops would be a poor decision. These troops for the most part should be highly mobile and divided into small groups with easy routes of disengagement. If they are ever being overwhelmed, they should retreat and regroup in a safe area. Losing the troops to overwhelming odds or risky maneuvers is pointless.

M4 Soldiers


The modern infantry men with M4's would be a huge asset seeing as most of the combat of the time was en masse. Especially since they are trained as such, I would use squads (9 or 13 depending if they are Army or Marines) of modern soldiers as mobile, long-range guerilla fighters.

They should be mounted if possible to allow quick transportation to different areas of the battlefield. Each squad could then engage massed formations from defensive positions at long range (250 - 500m). The accuracy and fire rate of an M4 would allow the squad to incapacitate entire formations at a time, likely with such speed that the enemy would not be able to return more than a volley or two of fire. Such a squad would likely suffer little to no casualties, especially with modern body armor.

None of these squads need to have a SAW or any snipers, though deploying them that way could be effective. Realistically, the firepower is already overwhelming, reducing the need for SAW suppression, and DMR configurations of the M4 would be effective enough to fit most front-line "sniper" roles. Not to mention that the Barret .50 is not as well suited for anti-personnel as an M4, especially when you are in open engagements with rather exposed targets.

If you are worried about any of your officers or other officials, a squad of modern soldiers is the best security you can provide.

M249 SAW Soldiers


In a civil war setting a SAW is an ideal defensive weapon. Used against approaching troops in tight formations it would have incredible effect, and could do so at distances well out of black powder rifle range.

The reliability of these weapons is unmatched by civil war era armaments, and the ability to fire long bursts at high rate was unheard of (the closest equivalent was a gatling gun, which was rare, not reliable, and could not sustain fire for very long). Just be absolutely certain that you do not overheat the barrels to extend their life as much as possible (unless new barrels are lumped in with the unlimited ammunition supply).

The suppressive ability of a SAW, and the fact that they can move forward with counter-attacking troops means defense would be very difficult to penetrate, and enemy advances would be stalled significantly. The suppression would also allow supporting troops (like your mobile M4 infantry) to flank and maneuver much more easily.

Barret .50cal Soldiers


These weapons were intended as mobile anti-material platforms. They can destroy light vehicles, penetrate soft cover like plaster, brick, and wood barricades, and have incredibly long effective ranges. These troops should be deployed to take advantage of these capabilities.

A few of these troops as guerillas could decimate the enemy's best weapons: artillery batteries. Their rounds are also ideal for destroying wagons and horses used in supply lines. Entrenched troops in defensive positions could be drastically softened before attacks as well.

Perhaps the greatest advantage (and other posts have covered this very well) is targeting enemy high ranking officers. You could eliminate officers efficiently from ranges well outside of engagement distance for standard troops at the time. As stated before however, you should try to organize a mass targeting of officers the first time the snipers are deployed, as after that, the officers would be much more cautious.

Summary


Your advantages in effective range, rate of fire, destructive capability, reliability, mobility, and survivability (body armor and range) will be incredible. You need to make sure these troops are deployed effectively and use them to end the war.

  • Use these troops in a very decisive battle where the enemy is goaded into overextending
  • Use the SAW's in a support role to strengthen defensive positions and provide suppression for your other modern units to maneuver.
  • Then deploy the other troops; largely as mobile cavalry guerilla fighters.
  • Use them against high value targets like officers, artillery, and supply lines
  • Where needed, deploy against massed troops to change the tide of battle
  • Do not over-extend any of these troops as each troop or weapon is incredibly valuable, and you want to keep the advantage over time.
  • If needed, a small number of troops can be used as security forces.

NOTE

The M9 pistols are relatively inconsequential seeing as almost every engagement in the American Civil War was open and at ranges much more appropriate for an M4; not a lot of urban or indoor warfare where a pistol might be more effective. Also, until very recently, it was policy to not issue side arms to most infantry soldiers. Regardless, they could give their pistols to regular union officers, which would be very effective compared to the black powder revolvers of the time, even without training. Having the pistols as a defensive weapon for your snipers is another effective use.

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    $\begingroup$ Yes, that’s exactly what happens to their pistols, they’re given away to Officers while the soldiers keep their rifles $\endgroup$
    – DT Cooper
    Oct 6, 2021 at 22:31
  • $\begingroup$ It's worth noting that the American troops gave officers targeting priority during the American Revolution, to great effect. With modern troops and weapons? I shudder at the thought. $\endgroup$ Oct 7, 2021 at 11:09
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They'd be pretty lethal on a tactical level. One of many reasons why World War 1 was so grotesque was because soldiers (towards the beginning, at least) lined up and walked/charged into machine guns. Imagine that but with less advanced military thought/planning and more powerful weapons - it would be horrendous carnage. Once you've machine-gunned a fifth or so of a formation, the remaining four-fifths tend to break and run, and the sight of tens to hundreds of men being cut down like wheat would annihilate morale.

Tactically speaking, they'd be a great anti-breakthrough unit: firing from concealed positions against incoming charges and making them turn and run. If they get exposed, they'll likely be killed eventually, so they're less effective on the offensive.

However, they're probably less dangerous in this regard then they would be if they employed themselves on a strategic/logistics level.

Tactically, they can't be everywhere at once. Strategically, they can increase the Union's strength enormously.

They can train other soldiers in things like modern first aid, squad tactics (good for urban fighting, if not combat on an open field), and modern logistics. Those soldiers go on to train other soldiers, and so on and so forth. Training them in modern hand-to-hand combat would also be quite useful; modern soldiers receive training in this, whereas Civil War ones basically brawled.

They could each be attached to Union generals as tactical advisors. They know what the Confederacy is going to do, to an extent. They can encourage, say, the Anaconda Plan, knowing that it'll work, among other things - basically, they're a source of info on what the Confederacy will try later in the war, meaning that the Union essentially has clairvoyance.

They can stop Lincoln - their commander in chief - from being assassinated, ensuring that Radical Reconstruction happens, which, in my humble opinion, will do a lot of good for the US further down the line. Best president ever GG no re.

They can use their radios to drastically beef up the Union's logistics and communication for as long as the batteries last. If they have hand-crank generators, that's even better, because they'll last until they wear out, and spare parts might actually be producible, given that, at the time, electrical power had a technology readiness level of about 6, rather than 1.

Their technology might be somewhat reverse-engineerable; the Union's finest gunsmiths might be able to produce a rudimentary semi-automatic weapon, although it wouldn't have much of a service life, and reliability would be an issue - both due to the constraints of engineering and science at the time.

They can inject a lot of modern knowledge into the US at the time - for instance, "here's how you treat a sucking chest wound", or "don't let lead get in the water", or "electricity is something you should put a lot of research into".

In short, they're much better at making the entire Union hit harder than they are at actually fighting, and people will believe everything they say once it turns out that they can basically predict the future.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Oct 5, 2021 at 12:33
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    $\begingroup$ Radical Reconstruction did happen. Lincoln was opposed to it as he wanted to peacefully reintegrate the southern states back into the union. The biggest problem is, when the Radical Reconstruction was going on, there was no political will to halt the gowning resentment of southern white from undermining all the civil rights gained by marginalized peoples in the south. They basically sat back and watched the KKK dismantle all the progress made in the first 10 years of reconstruction. $\endgroup$
    – Sonvar
    Oct 5, 2021 at 20:59
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Communications!

The answer is in this sentence: "They have advanced weapons and tech"

Split your modern troops up 1 to a Corps, or 1 to a Division if you can get enough radios. Reserve a handful of radios for advance scouts and lookout posts.

The simple ability for an army in the field to receive instantaneous communications about its situation and orders, to coordinate attacks, to send reinforcements at need, to dynamically react to events on the battlefield outside of line-of-sight, will be a much greater force multiplier than any actual combat role you could conceive for your modern troops.

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Hospitals/ Battlefield medics

Some basic googling suggest maybe half to 2/3 of people died from infectious diseases. I expect even basic knowledge of things disinfecting water, disinfecting wounds etc would save many lives. Modern medical supplies would be limited and run out quickly but even the basic ideas about first-aid and keeping things cleans would give a much higher survival chance to individual soldiers and thus larger army to fight with. If there are any doctors or other well trained medical personnel in the group they could probably help train the local doctors in more modern technique (things like better amputation techniques, surgical procedures, wound management etc)

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    $\begingroup$ This. Germ theory. Using heat or alcohol to disinfect things. Maybe even producing antibiotics from Penicillium moulds would be ground breaking. $\endgroup$
    – Michael
    Oct 4, 2021 at 10:19
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    $\begingroup$ I was once at a Civil War reenactment where the doctor told us that wounds have to be moist to heal properly, so you take a bucket of water and a rag and just go down the line sopping everyone's wounds one after another. :O $\endgroup$ Oct 4, 2021 at 13:39
  • $\begingroup$ At this point in history more people died of disease than battle. Your chances of survival went down when you went into a first aid camp. But 3 companies worth of troops aren't going to have that many medics, it would take time for them to propagate that kind of knowledge throughout the Union army $\endgroup$ Oct 5, 2021 at 8:46
  • $\begingroup$ I am extremely skeptical that the proposed benefits - for meds - could be implemented. The time travelers would have to get the cooperation of medical and first aid staff of the day. AFAIC, highly improbable. $\endgroup$
    – Mark G B
    Oct 5, 2021 at 15:04
  • $\begingroup$ Rediscover penicillin? By knowing the 1% of science that works it saves having to do the 99% that failed. $\endgroup$ Oct 5, 2021 at 22:40
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Relieve Fort Sumpter. Capture General Beauregard.

https://en.wikipedia.org/wiki/Battle_of_Fort_Sumter

The ability of the Confederates to defeat the Union at Fort Sumpter encouraged their idea that they could pull off the secession. Your 300 arrive in Charleston and destroy the batteries shelling Fort Sumpter one after the other, leveraging the sniper rifles.

Your future soldiers (in Union uniforms, right?) then proceed to attack the assembling Confederate Army and capture General Beauregard. He will be released. He will be shocked and awed by what has happened, and bring his story back. He will note that the Union is apparently capable of far, far more than they realized.

Maybe the rebels will rethink their course of action.

Probably not. Americans are stubborn, Southerners more than the rest.

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    $\begingroup$ Most (all?) of the Confederate upper command were current or recent serving officers, to say nothing of many of the front line troops "having a difference of opinion"; they will be familiar with the standard weapons and tactics of the Union troops, because they were Union troops until extremely recently. They will know something is off essentially immediately (at minimum, that the weapons used are not standard issue). Attempts to thwart the fall of Sumter may still succeed, although disabling the attacking guns will not be as easy as you imagine (since most of them will be protected). $\endgroup$ Oct 4, 2021 at 16:37
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    $\begingroup$ An excellent use! The attack on Fort Sumpter was reported in near-real-time by a large press corps. I think the Union's well-observed and surprising capability, invincibility, and speed would indeed shock many in the CSA into reconsidering their course of action. $\endgroup$
    – user535733
    Oct 4, 2021 at 17:28
  • $\begingroup$ @Clockwork-Muse I think soldiers from any era would find plausible that there exist special soldiers with special abilities and equipment, deployed in special situations. There really are. Usually not futuristic supersoldiers but the attack on Osama Bin Laden was pretty close. $\endgroup$
    – Willk
    Oct 4, 2021 at 21:29
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    $\begingroup$ @Willk - Special abilities and equipment? Sure. But even with the modern penchant for secrecy that doesn't usually cover infantry weapons, and many of the commanders were or had been in positions where they should have been "read in". There's also too many troops so armed for this to be a special unit. You're expecting the Germans before WWI to believe every US plane is a P51, but all their intelligence services and engineers are going to be telling them, "the US didn't make this, nobody can make this yet". You cannot surprise people with this level of technological change. $\endgroup$ Oct 5, 2021 at 3:37
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    $\begingroup$ @ Clockwork Muse A lot of Reber officrs were US Officers who resigned. But the Rebels raised armies which eventually totaled close to ten times the size of the entire prewar US army, enlisted and officers combined. So the vast majority of Rebels, including officrs, came from civilizan life. And most of the enlisted men in the US stayed in the army, so there were few of them in the Rebel ranks. But you are correct that nobody would think that most Union soldiers and their equiplment could be so radically superior as the ones from 2021 were. $\endgroup$ Oct 6, 2021 at 2:03
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Decapitation strike: 1854

The Civil War began with a secret society, Knights of the Golden Circle, which organized a series of "filibuster" campaigns, including the temporarily successful conquest of Nicaragua in 1856, before bringing their troops home to form the nucleus of the Confederate Army. Given time travel, you should set your mission to target their first "castle" of five men in Cincinnati in 1854 that began the organization, or William Walker's force in 1856 that popularized imperialism in America for the following 167 years and counting.

Of course, the premise of stopping the Civil War earlier in any way carries with it the risk that the Emancipation Proclamation would never have been issued, and that in a spirit of compromise, progress against slavery might have been even slower and more miserable.

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    $\begingroup$ This does nothing at all to address the fundamental conflict: the South's economy and its elite's power and fortunes depended on slavery, but newly admitted states were going to result in an anti-slavery majority in Congress pretty soon. Thus your idea would most likely only delay the civil war. One might argue, though, that the later it happens, the quicker and easier the North's victory would be, due to its economy being accelerated by industrialization. $\endgroup$ Oct 4, 2021 at 11:28
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    $\begingroup$ Past causality (a fixed past and undetermined future) isn't really true, but if it were, then I admit any manipulation has highly unpredictable effects. (And otherwise, very little effect at all) What you say is certainly possible, yet we see from current events that vast multitudes of people can work vehemently against their own financial, political, or even biological interests. Additionally, outside the elites, many people saw little benefit from competing against slaves. But although the effect would be unpredictable, fighting history's villains might at least be satisfying. :) $\endgroup$ Oct 5, 2021 at 0:02
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Propaganda

Around a third of those 300 will be non-white, with white enlisted people serving under non-white Officers and NCOs.

Stage demonstrations of the visitors massively superior military power to back the claims that they are from the future. State that the North wins the Civil War, even without the visitors help. Make the claim that in the future slavery is dead, and that inevitable technological advances will render slavery economically inviable within a generation. Point out that the visitors are assisting the North in the production of their weapons and soon every Union trooper will be armed with an M4. (This last is an out right lie, the best they could hope for would be rifles like the Lee Enfield, Mauser 98 or 1903 Springfield. Reliable machine guns more advanced than the gatling gun will be out of the question without good smokeless powder, but hey! its propaganda, not truth, and bolt action rifles with jacketed bullets will shoot rings round rifled muskets with Minne balls in any case).

Internally convince the North that the only way for the US to take its future place as a Global Superpower is to reunify as rapidly as possible and to really commit to taking the secession seriously and recruiting and training a large effective army. Make the enemy the Rich corrupt land owning slave owners who oppress white and black alike and make it a war of liberation of the South.

A better prepared North that can credibly claim guaranteed victory (based on future knowledge) might collapse Southern morale so completely it leads to surrender.

Also of critical importance is to ensure that Northern politicians understand the necessity of winning the Peace. The victory in the Civil War was sufficiently decisive to prevent any further insurrection. In the absence of that it has to be clear that being in the Union is way more advantageous than being out of it to prevent a second revolt when the Southern states catch up with the North's new military technology - which is inevitable. The first self loading rifles will be developed within 50 years even without people familiar with their design and operation and modern examples to copy. Invest in the economic development of the South without slavery and make a lot of noise about doing it.

Edit

Prompted by The Daleks comment I have given it some thought, and in fact I have thought of a design for a machinegun that I believe would operate reliably with black powder ammunition. Moreover, since I came up with it in minutes of actually thinking about the problem, I believe it would be obvious to anyone with a reasonable knowledge of historical firearms.

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    $\begingroup$ It's possible to make machine guns without smokeless powder. Using black powder only means that they'll have to use blowback mechanisms (commonly used in pistols and submachineguns) instead of gas-operated ones. They won't be as accurate or long-ranged as a smokeless powder machinegun, but that doesn't really matter in this scenario. $\endgroup$ Oct 6, 2021 at 14:15
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    $\begingroup$ Oh yes, I totally agree that you could build recoil operated machine guns. I'm just not certain of their reliability. Black powder fouling tends to get into the entire mechanism. One of the problems several of the early breech loaders had to overcome was that of the action becoming too stiff to open/close after prolonged firing due to fouling. $\endgroup$
    – ShellGhost
    Oct 6, 2021 at 14:34
  • $\begingroup$ @The Daleks, see edit. I'd upvote your comment twice if I could. Thanks! $\endgroup$
    – ShellGhost
    Oct 7, 2021 at 7:51
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The best possible use for them is to not use them at all. Get them off the battle field and hide them somewhere harmless until they can be returned to the future.

Travelling that far back in time with that much weaponry has the potential of totally disrupting the existing timeline. Grandfather paradoxes, unexpected survivors and the acceleration of the Union victory by several years, the American population following your marines deployment will have no relationship to the population which survived the original conclusion.

Any use of such a powerful force during this pivotal moment of history would have vast effects on the entire future. Given how close we've come to self-annihilation during the final decades of the 20th century, we should be very careful inciting any temporal aberrations prior to the cold war years. A different set of our parents might have to live again through that entire accursed time, and this next time, none of us might come out of it so lucky.

Kill the Time Travelers! Protect the Present!

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  • $\begingroup$ Or might turn out better. You just cant tell.... $\endgroup$
    – Stilez
    Oct 4, 2021 at 7:10
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Frankly the most dangerous weapons 20th century solders would have are not guns.

I'd say the best/most dangerous weapons are radios and communications. Modern infantry units would have UAVs (and short of massed fire, its unlikely civil war era troops could take them out. )

Even a 'weak' radio with say a 4km range would let you get an hour or so's warning out at 'modern' marching rates.

Even with weapons of the era, knowing where the enemy is, and knowing how effective your fire is would be invaluble. One soldier reporting an enemy column is moving a certain way means a whole load of his side, either with contemporary or modern weapons know where, when and how to ambush them, and know how many of the enemy they have.

This would be a massive force multiplier

I'd use 'modern' troops as raiders - hitting soft targets and resupply wagons. Shoot horses, kill guards and either take or destroy the enemy's supply lines, or to decapitate enemy generals

I'd also consider the impact of other modern equipment, body armour is likely to easily stop lead balls, modern helmets...

And Night vision - since your troops can see the enemy in the dead of the night, and attack enemy forces unseen. Being able to sneak into an enemy base, and tamper with supplies or slit random guard's throats is likely to have an outsized reaction compared to the more energetic ways of breaking things and killing people.

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Background

Before we answer the question of: "How best to use modern troops?" we need to understand the fighting conditions of the day. One of the most important facts is the engagement distance. Although both sides had rifles with a theoretical effective range of 400+ yards, most engagements happened at 50-100 yards! There are several reasons for this fact:

  1. Gunpowder was a severely limiting resource. Many soldiers were not proficient marksmen because neither army could produce gunpowder at a sufficient rate to allow them extensive shooting practice. Some soldiers were observed shooting their rifles at a 45 degree angle into the air, for lack of actual marksmanship experience and understanding of ballistics!
  2. Black powder turned the battlefield into a giant smoke cloud. Smokeless powder was one of the most important innovations in warfare, because it allowed soldiers to see the enemy even after many rounds of rifle fire. Soldiers often closed to 100 yards or less simply because they could not see what they were shooting at in 400 yard rifle range.
  3. Optics were primitive. Iron sights were generally the only optics available, and a lack of practice shooting means that most soldiers did not know how to properly account for bullet drop over longer ranges. In the 50 yard range for typical engagements, bullet drop was not an important consideration.

Now, cannon were available, and although they had an effective range of a mile or more, they were typically used at 1000 yard ranges or less, due to accuracy and other battlefield conditions. But you have to realize that artillery pieces were massive resource hogs, often being manned by teams of 10 soldiers, multiple horses, and special wagons called "caissons" which carried the ammunition and powder.

In addition, both sides sometimes employed war balloons to provide forward observation for artillery targeting. Obviously, this provided a significant advantage to look behind the smokescreen created by one's own rifle line, as well as identify targets of opportunity behind the main line.

Orders were typically given verbally to infantry units, and by battle courier to officers and NCOs.

Equipment

You said that the modern soldiers just have "basic" gear. Given the facts above, I would strongly suggest that you equip them with the following gear, which is more or less "standard", depending on the unit (but none of it is "exotic"):

  1. ENVG-III, with thermal imaging which can see through smoke. This one piece of gear can be a total game-changer.
  2. ACOG sights, which are designed for effective targeting out to 800 m. No Confederate officer will expect a Union regiment to fire on his troops at 600 yards. At least not with any effect. Your riflemen with their M4s + ACOG will effectively all be snipers under Civil War battle conditions.
  3. M203 grenade launcher. This turns every M4 rifleman into a grenadier.
  4. M18 Claymore mines. Optional, but a massive force multiplier if used well.

Defense

One of the most effective use of the modern soldiers is defensively during a large pitched battle. You'll want all of the troops to be equipped with the thermal imaging goggles for battlefield visibility. That's expensive, but you only have 300 of them, so no problem, right? I would then assign targets as follows:

  • Snipers: target balloons, then officers/color teams, then artillery
  • SAW gunner: start fires at 6-700 m starting at the flanks and working towards the center until enemy is defeated
  • M4 riflemen: engage single targets with precision fires (single round at a time) at 500 m, then switch to M203 at 350 m if there are still massed troops approaching. Otherwise, continue with precision fires.

Before the battle, you want to prepare the field with claymores, if you have them. They should be placed in the middle of each formation where enemy troops are expected to charge, about 150 m from the front lines (i.e., before the typical engagement distance). It's more effective to place them densely in the center rather than spreading them out, for reasons which will soon be clear.

When the fighting starts, the SAWs will start picking off troops at the flanks of each formation. The enemy will quickly learn that that is a dangerous place to be, and will charge towards the center for "safety". Union soldiers can make a show of faking casualties towards the center, or even fleeing it for the flanks, to make it look like the Confederates will be able to break the lines if they push forward. Warfighting theory at this time was heavily predicated on breaking through enemy formations to get behind the lines and wreak havoc, so this will be a natural instinct for many soldiers.

Of course, what we are really doing is creating a kill box for the M203s and the claymores. The SAWs should be able to thin a pretty good number of troops in the 500-800 m range. They are modern weapons designed for the modern battlefield where suppressing enemy troops hiding in cover is the task of the day. Against unarmored troops charging in the open, they should simply annihilate any charge almost by themselves. But let's say there aren't enough to finish the job by themselves. The M4s should be able to lay down effective and deadly fires in the 350-500 m range, thinning the charge dramatically. At at 66% accuracy, the riflemen should be able to hit about 20 soldiers per magazine, or about 140 per 7-mag loadout. The SAWs at 50% accuracy should be able to pick off about 100 soldiers per 200-round belt of ammo, or 400 soldiers for a typical loadout.

If we assemble 10 squads of 8 riflemen + 1 SAW + 1 sniper, then each squad should be able to average maybe 1500 kills each. Now, that's only counting rifle kills. If we consider each M4 rifleman carries 4 grenades with a 130 m casualty radius, and assume soldiers are bunched up at 2 m apart, then each grenade may be able to cause 60+ casualties, or 240 per loadout. That's another 1920 casualties per squad. If we add 2 mines for every soldier, and we assume each mine can injure another 25 enemies, then that's another 400 casualties per squad. That's nearly 4000 casualties inflicted per 10-man squad!

The average Confederate brigade was a force of about 5000 soldiers, so given that some soldiers will inevitably break and run, we can safely surmise that a single well-armed squad could defeat an entire Confederate brigade all by itself. If you add on Union soldiers picking off the stragglers, it seems likely that a squad + Union brigade could easily take on 2-3 Confederate brigades.

I say the Union soldiers would become far more effective because the modern soldiers would be firing on the enemy and inflicting casualties long before the enemy got into typical engagement range. So the Union soldiers could mostly hold their fire until enemy troops actually closed to the 50-100 m range, giving them a clear view on their side. The Union also had superior artillery support, and could unleash devastating volleys at the remaining attackers.

A better use of Union soldiers would be to push them forward in a skirmisher line. They could engage from a prone position all the way out to the line of claymores (which obviously they want to stay behind), making their effective range about 200 m. They should stay mostly on the flanks to funnel attackers into the killzone, and keep their heads down so the modern fighters can shoot past them (although, the Union often had the high ground, which makes shooting over them much easier).

Offense

After some devastating defeats using the tactics above, the modern soldiers should strike out on their own as a few special hunter-kill companies (up to 5). Catching a Confederate division on the march with the cover of forest would very likely cause them to scatter and flee, leaving behind their artillery and most of their supplies. Since the Confederates were already struggling with supplies, this kind of harassment would likely deplete the warfighting capability much faster than open combat. There is no evidence that Civil War era soldiers had any experience in insurgent-style ambush tactics or warfare, and commanders would struggle to organize their troops under such conditions. Again, claymores and snipers can be used to great effect by preparing the battlefield and targeting officers, color guards, even horses. Artillery can be rendered completely useless by lobbing a few M203 rounds on the caissons.

But hey, why expose yourself to enemy fire by attacking in the day? We've already equipped the entire force with NVG, so launch the raid at night, starting with that artillery powder, to make some nice bright startling explosions to wake everyone up at 2 AM. Even modern militaries know that the US owns the night, and that is when they are most vulnerable to attack. A Civil War era general will literally not know what hit him and will struggle to mount any kind of effective defense. It would pretty much be shooting fish in a barrel. At that point, let the M249s loose on rapid fire and don't worry too much about conserving ammo. Most soldiers would probably flee in the night hoping to escape with their lives. Let the general fight the demons and ghosts who can see in the dark!

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Hot Air Balloons - Sniper Rifle Edition

Balloons were first used for military observation in the 1790s, and were in fact used during the US Civil War.

Nothing in the Rebel Army is going to have the range / stopping power to damage a balloon in flight unless they get very close.

It's going to be extraordinarily difficult to get close to the balloon when there's a bunch of Union soldiers manning the tether at the base, and a sniper in the balloon acting unopposed.

From the elevated position, with vastly superior range, the snipers can systemically dismantle the Rebel Army. When you kill the Rebel officers, runners, and scouts, their force ceases to be an army, and becomes a mob.

Strike at the Heart

American Generals were not aggressive enough early in the war - Having 25 SAW machine guns is going to change that. American forces could strike at the major East Coast cities; starting with the Confederate capitol in Richmond.

Break the modern soldiers into groups, and pair them with the normal Civil War soldiers with a ~25:1 native to modern ratio.

Have a small group of the modern soldiers oversee protection of the 25 Sniper Balloons, and the remaining 200 or so would be organized around the SAWs. The snipers and traditional scouts find the largest concentrations of Rebel soldiers, and they direct the SAW squads towards them, and the SAW squads will do the whole "close with and destroy the enemy with fire and maneuver" thing.

The Rebels could learn to deal with the SAWs or the snipers, but not both. Not fast enough to matter. The first battle would be a bloodbath - the snipers wreck communications, and the SAW-squads wreck everything else.

There probably wouldn't be a second battle. The Rebels wouldn't be able to put the army back together again fast enough. American forces advance on Richmond, occasionally harried by small Rebel groups, but effectively advancing unopposed.

When Richmond falls so early in the conflict, all hope of international recognition is gone. The Army of the Potomac prepares to move further south, but Rebel leaders formally surrender to American forces before it has a chance to move out.

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There is a lot in their training that puts them ahead of the generals of the day in terms of military theory. They know how stealth improves survivability when fighting with firearms and how to get the best out of dead ground etc... They know a lot of tricks for intel gathering etc... that are relatively recent inventions that will still work in the American Civil War. You could do far worse than putting them into the training corps.

They also have a working knowledge of a number of other weapons they don't actually have with them (in particular mortars and recoilless rifles are very simple weapons that can be built with bronze age metallurgy as long as you know how to make gunpowder; they are well within the reach of the gunsmiths of the Civil War) so R&D is also a useful role.

They should also know the history so to begin with, if they're on the planning staff, they can out maneuver the Confederates, that's an advantage that won't last more than at most 2-3 engagements so use it wisely.

If for some reason the brass insist on putting such valuable sources of knowledge on the front line then the best use for them is as a deep penetration asset operating behind the enemy lines attacking soft targets like supply dumps and manufacturing centres. Also as pro insurgency forces supporting slave desertions and uprisings on southern plantations.

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    $\begingroup$ Modern training would not be such a big advantage. Any training is based on available equipment - both to you and your opponent. A lot of this training is based on availability of GPS, detailed maps, air support, motorized transport and such. Unless we are talking about Green Berets or or Navy SEALS, there is not so much they can teach in terms of stealth and survival. $\endgroup$
    – Alexander
    Oct 4, 2021 at 18:02
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    $\begingroup$ Mortars can be built with bronze-age metallurgy, but they're not very effective without explosive shells. During the American Civil War, shells were still very much an experimental technology. $\endgroup$
    – Mark
    Oct 4, 2021 at 21:22
  • $\begingroup$ @Alexander The Civil War was largely fought by standing soldiers on open fields, even in cases of static defense, because it was the way infantry had fought when guns weren't a factor on the battle field it wasn't until after the mass casualties of the open battles of the First World War that the attitude that glorified "meeting the foe" went by the boards, and modern soldiers already have that advantage. $\endgroup$
    – Ash
    Oct 5, 2021 at 5:18
  • $\begingroup$ @Mark Impact detonated shells were still highly experimental, time fuses were already a mature, if still unreliable, technology from the age of sail. $\endgroup$
    – Ash
    Oct 5, 2021 at 5:20
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    $\begingroup$ @Ash, "standing soldiers on open fields" went by the wayside less than a year into the Civil War -- General Lee may have gotten the nickname "King of Spades" as mockery of his trenches around Richmond early in the war, but it stuck because the Army of Northern Virginia had a habit of digging field fortifications given the least chance. Compact formations stuck around longer because until you invent the magazine-loading rifle, you need dense formations to stop cavalry charges. $\endgroup$
    – Mark
    Oct 5, 2021 at 21:00
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We know that the Union won the civil war.

We do NOT know the effect, if any, of applying time displaced troops from the future back to the Civil War.

The wise move would be to avoid loss at all costs by keeping those troops sequestered.

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Tactically?

Let’s see how a modern soldier matches up against a Union infantryman.

The 1861 Springfield rifle (https://en.m.wikipedia.org/wiki/Springfield_Model_1861) is a 0.58 caliber weapon, with a muzzle velocity between 1,000 and 1,400 feet per second, an effective range between 200 and 400 yards, and a rate of fire of 2 to 3 shots per minute.

The modern M4 (https://en.m.wikipedia.org/wiki/M4_carbine) fires a roughly 0.22 caliber shot at around 3,000 feet per second with an effective range of about 550 yards, and theoretically can lay down 750 to 900 rounds per minute, but is practically limited by 30 round clips.

Comparing :

Damage — Union infantryman

Range — Tie

Rounds — Modern infantryman

These soldiers are embedding in forces (if part of the Army of the Potomac) about 100 thousand strong and engaging in conflicts that leave about 40 thousand casualties.

Although the modern infantryman is more individually potent (maybe 10x times as effective as his Union peer), accounting for the technology force multiplier would make the force “effectively” 3,000 men, or 3% of either side.

There’s no particular front-line role where they would seem to shine. I’d imagine they would be an individual unit, to be used as best as possible.

Now, I let’s take a look at those heavy weapons

The M249 (https://en.m.wikipedia.org/wiki/M249_light_machine_gun) fires the same 0.22 caliber round as the M4 at the same approximate 3,000 feet per second velocity, has a slightly better effective range of 700 yards, and the same theoretical firing rate, but is practically limited to about 100 rounds per minute.

Accounting for force multiplier (100x), 25 of these things are worth about 2,500 of their peers.

The whole modern combined arms force, then, is about as effective as about 5,500 of their Union comrades. In these larger campaigns, in my mind, there is no role that they stand out for. I’d still think they are one frontline unit among many.

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    $\begingroup$ No mention of those snipers, tho' $\endgroup$ Oct 4, 2021 at 2:55
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    $\begingroup$ Energy of a bullet is M(ass) * V(elocity) ^ 2(squared), so a larger caliber, heavier bullet is not necessarily more damaging (although the modern round likely loses some effectiveness due to it often being armor piercing - troops of the time didn't wear significant armor - and passing completely through them means it probably transfers less energy to the target). $\endgroup$ Oct 4, 2021 at 16:48
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    $\begingroup$ @Clockwork-Muse Passing through the soldier means an injured soldier not a killed one. An injured soldier ties up 2 more soldiers in getting him to care. $\endgroup$ Oct 5, 2021 at 2:38
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    $\begingroup$ There are a couple key points about the M4 that are being overlooked here. First, its design is based on studies that showed that the amount of ammunition carried was more important than the caliber of hits; that was the reason for the switch from .30cal infantry weapons to .22cal infantry weapons after WWII—more ammo, more effectiveness, despite the smaller round. Second, the M4 is designed to cause the bullet to tumble in the body, dramatically increasing its effectiveness. I suspect the M4's 5.56 bullet would actually do more damage to the body than the 1861 Springfield ball. $\endgroup$ Oct 5, 2021 at 16:35
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    $\begingroup$ @Clockwork-Muse Passing through a soldier could, at this time, mean very easily passing into another soldier, lined up behind him, perhaps more. With the 150 metres range advantage, they would mow them down like grass. $\endgroup$
    – Eleshar
    Oct 6, 2021 at 8:46
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I feel like there's some overthinking going on here. A squad with that force who was even moderately trained going into the (first) battle of the bull run would have utterly obliterated the opposing army without even breaking a sweat. Each side was completely unprepared and untrained at that point, fielded about 18k soldiers each and lost about ~400 each. You'd be past 400 within seconds of engagement and it's not unreasonable to think, given the tactics used, nearly 100% of the opposing army would be destroyed if the attack was timed correctly. It would make a WW1 trench charge under heavy machinegun fire look like a pleasant walk through the park by comparison. Such destruction would be about the equivalent of the atom bombs in terms of how quickly surrender would occur afterwards.

I'm sure this might come across as an overexaggeration, but it's important to look at how armies formed themselves, particularly in the early battles. You had thousands of soldiers in tightly packed lines, complete sitting ducks for modern machine guns. Their leadership would have been immediately blown to pieces by the snipers, leading to terrified mass confusion.

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Don't use them. Use their knowledge.

Firstly and most importantly as someone else has mentioned, it is essential to keep them and the fact of their arrival secret and hidden. This is because their knowledge is the key to victory not their weaponry. There are supposedly only 300 of them even of they do have 'unlimited' ammo.

(How does that happen BTW? If they have unlimited ammo that implies a continuous and ongoing time travel capability which presumably also means there's nothing stopping more troops equipment and supplies being sent.)

But back to the point. There's only so far you can disburse that much firepower without diminishing its impact/utility. And you have to remember that by the middle of the war both sides had access to rifled muskets with effective ranges out past 1000 yards! Even civil war carbines were effective out past 300 yards and the 'enemy' had tens of thousands of them. Plus in theory there's nothing stopping the enemy from 'winning' any one battle simply by virtue of being victorious someplace else on the field your men aren't.

Also the first time they engage in a historical battle? That changes the dynamics of the war. The battle and its outcome will no longer the one they remember and the one after that will entirely different again. So their advance knowledge of where best to position themselves to ensure victory at any original battlefield will also go 'poof' as the time line changes.

So back to their knowledge. If they're arrival remains a secret the odds are high that their collective knowledge of the Civil War (particularly that of the officers and senior NCOs) will let them give Lincoln factual assessments of every battle ever fought. Not just locations and outcomes but details of the mistakes made by opposing commanders and assessments of each commanders performance. So Lincoln has the chance to promote the best and remove the worst leaders well in advance of their victories and defeats.

They can advise the Union of where and when the South will be weakest and assist with technical advances just by suggesting improvements in how the North uses current medical, and engineering techniques. In short they can make the Union Army 'better' just teaching the North how to select and use the best of what they already have available without waiting for 'miracle' weapons from the future to arrive or be built.

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  1. Communications: give one to each general for direct, unbreakable communication
  2. Code breaking: use a pocket calculator or something to break the opponents communications, they seem trivial maybe will also need a bit of help from locals to understand the message as well, then have someone tap the telegraph line and directly over radio transmit enemy communications.
  3. Have someone observe army movements and directly report them via radio (do they have any infrared...)
  4. Have someone with a pocket calculator and/or knowledge about logistics provide better logistical support to predict what is needed and where... Logistics...
  5. If you need something heroic and decisive, and using info from radio have them take down an enemy general while he moves from one place to the other

But also, Use some modern sense and arm some black men early on...

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Full Metal Jacket:

There are plenty of solid answers here, but almost nothing has been said about logistics and technology transfer.

The biggest changes that could be wrought on the Civil War would be in the supply of advanced guns to the North and the denial of guns to the South. This would involve a mix of tactics and logistics.

  • Arrive at Harper's Ferry: Most of the equipment used to arm the South was made with industrial equipment captured by the South at Harper's Ferry. Without this equipment, the Southern forces would be mostly armed with smoothbore muskets. The actual Southern force that seized the armory was trivial - about 360 men. At the start of the war, the South had 175,000 'modern' guns, but 140,000 of these were smoothbore. 15,000 of those rifles were stored at Harper's ferry itself. Denying the South the ability to produce rifles would cripple their ability to maintain the war effort and seriously degrade Southern arms at the start of the war. There would need to be a stronger blockade of Southern ports to prevent European guns from coming in.
  • Move the armory to safety: All this stuff would be valuable to the North, but especially for my other plans.
  • Make repeating rifles with smokeless powder, metal cartridges, copper jackets, and repeating actions: A sniper rifle is going to contain the basic engineering and designs to produce a magazine-fed bolt action rifle. Most of the mechanics of a bolt action rifle were already invented and in use in Europe. The Union actually fielded 900 at the battle of Antietam, but they decided they were to complicated to maintain and operate for regular soldiers. With the use of smokeless powder, metal cartridges and copper-jacketed bullets (a “full metal jacket”), the design and maintenance deficiencies of bolt-action rifles are solved. The technology to produce both smokeless powder and electroplated bullets existed at this time, but the ideas hadn't been developed. A Union army deploying these rifles (easily used with rapid fire in a variety of positions) wouldn't need time travelers shooting guns. Even if the South captured some, they wouldn't stand a chance at making them.
  • Train the Northern army: making full use of more modern firearms will require different tactics. A small force trained to use cover, move and maneuver and just generally use more modern techniques with guns that fire fast, accurate and at longer ranges would decimate the South.
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  • $\begingroup$ 300 soldiers are not a single gunsmith. It takes more than knowing how to use a gun to know how to manufacture it. $\endgroup$
    – Trish
    May 5, 2022 at 6:03
  • $\begingroup$ @Trish They don't even need to know anything. The gunsmiths are there, the North was already considering a bolt action design, smokeless powder and a full metal jacket just needed the idea of gun cotton and electroplating bullets. They are the inspiration, examples, and the foreknowledge of what the future holds. $\endgroup$
    – DWKraus
    May 5, 2022 at 10:51
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Capture the Flag!

NOTICE to the Downvoters: The OP asked for 300 US Soldiers or Marines with "advanced weapons and tech". Military technology includes not just weapons, but transport, communications and intelligence. DT Cooper, the OP, asked for advanced tech and I supplied the modern US Navy in the form of an aircraft carrier, and perhaps some support vessels, along with the temporal portal to get them to the 1860s. If you downvoted because you don't like the answer, please ask a different question and be more specific about what you'd allow!

Since they have "advanced tech" at their disposal -- I'd say just pick a nice day to drive an aircraft carrier up the James River, send out some planes to lay down perimeter fire around Richmond, and then send those 300 US Marines in on helicopters or something to capture the C.S.A.'s Capitol, Bank, the other chief government buildings, main printing offices, post offices and telegraph offices.

Raise a 50 Star US flag over the Capitol and, having rounded up the entire Confederate government, set them to surrendering because the gig is up.

I can't see wasting our modern tech and training plus the ability to send it through time on sitting these men up in Antietam or someplace like that, where others have said they'd be, at best, just another unit, which means cannon fodder with fancier guns. Let em hit the heart and hit it strong.

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    $\begingroup$ Nope not posssible. They don’t have Aircraff carriers, just the 300 troops with their weapons. That’s the rule of the hypothetical $\endgroup$
    – DT Cooper
    Oct 4, 2021 at 1:17
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    $\begingroup$ The Capitol was already in the hand of the side they are supposed to help. 34 states (thus stars) during Civil War. Are you suggesting a scenario in which... the nowadays-America invades the American-Civil-War-America and teach both sides a lesson in history-as-it-is-remembered-by-the-nowadays-grunts? $\endgroup$ Oct 4, 2021 at 3:08
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    $\begingroup$ @AdrianColomitchi -- No, in our timeline the Conferate Capitol was captured in 1865. In the alt-timeline, we're looking to capture it along with the government officials working in it rather sooner. Also, yes, the scenario as given is basically "nowadays America invades" the CSA: one of the the primary stipulations is 150-300 modern soldiers. $\endgroup$
    – elemtilas
    Oct 4, 2021 at 9:58
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    $\begingroup$ @elemtilas ah the Conferate had their own capitol. My "teach both sides a lesson" was a result of me thinking the one in WashDC was the intended target of the occupation. So, about those 50 stars US flag (as the 50-th was added only in 1962), does it mean the Confederate Capitol was to claimed by (in the name of) the nowadays US? $\endgroup$ Oct 4, 2021 at 11:49
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    $\begingroup$ The advanced tech that 300 modern US soldiers/marines might have with them would certainly not include an aircraft carrier, which has a crew of thousands. Helicopters or some other transport aircraft might be believable, and they'd be quite significant. $\endgroup$
    – Monty Wild
    Oct 4, 2021 at 23:55
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Filling the Hospital beds

These battle-hardened veterans face a few enemies they can not fight against, and the worst ones are plagues and malnutrition. While their brethren of the timeline are prepared and immune from some illnesses, these modern soldiers face the challenges of smallpox and dysentery as well as the very poor food support lines soon after they arrived.

Dangling at the gallows

These strange people wearing strange things must be invaders from far away. They speak a strange dialect and the flags on their shoulders are a mockery of the United States - clearly, they are traitors or want to take over the real United States! Hang them!

Dead like all the others

When it comes to an engagement, their superior weapons might allow them to hold one position quite well, but that will not stop the confederate troops from doing what they do: they will march thousands into the meatgrinder, try to outflank and encircle the position and then pour death into it from cannons and riflemen alike.

Even if this group is worth a whole 3000, maybe even 6000 soldiers of the union side, their ammunition will run out within a very short time. If they are present at Gettysburg, it would even be before noon of the very first day that they have run out of their bullets. Sure, the place they were would be a massacre, but then no bullet fly from the position because not a single cartridge is left while the rest of the troops on the battlefield, on both sides, has easily a few million bullets left and more bullets left - just nothing compatible with the modern guns.

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