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Problem: In a space-age war (with magic), why would ground troops be used, rather than just dropping orbital marines onto problem areas?

Elaboration: So to set the stage, we’re dealing with wars spanning many planets. Magic is also a thing, but I haven’t made strict rules yet, so you can be flexible with that (The only major rule is that using magic takes a toll, usually physical damage to your own body). Much of the time there will be a technology imbalance between foes, too. With all this in mind, when capturing a planet, what would cause the formula of “acquire a beachhead, then push with land forces” to need to be followed? Why couldn’t you just drop troop transports in free fall (sorta like in Starship Troopers) from the atmosphere onto enemy defenses and negate their lines?

The main thing I’m thinking of is that troop transports would still need to slow down so as not to kill the passengers, since magic would have a hard time mass producing enough equipment to counter that due to the physical toll. But it still seems pretty hard to counter pods of men smashing right into the middle of your lines at high speeds. Why would this strategy be ineffective, even against a technologically inferior foe?

EDIT: Man, I wish I could “accept” multiple answers, cuz both DWKraus and Palarran deserve it. Thanks so much to you both, your answers were super helpful and really struck at what I was looking for.

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    $\begingroup$ I apologize, but VTC:NDC. Why use ground troops in a space war that has magic but you've excluded magic as a requirement for the answer... I think? This question is, IMO, very unclear and incredibly vague. If you can bomb the planet with nukes, no troops are needed. If you want the surface intact, bomb with bio weapons or gas. It's almost impossible to completely rationalize ever using troops in interplanetary combat. And then there's all this undefined-but-intrinsic-to-the-question magic stuff. I recommend using the Sandbox. $\endgroup$ Aug 1 at 2:11
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    $\begingroup$ Do you mean an interstellar or interplanetary war? Intergalactic wars are much, much harder, and involve travel between or weapons fire between galaxies. Interstellar wars are wars between star systems, and involve travel between or weapons fire between star systems. Interplanetary war might be possible within the next fifty years. $\endgroup$
    – KEY_ABRADE
    Aug 1 at 3:43
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    $\begingroup$ Why aren't the defenders simply blowing the troop transports out of the sky with their own magic or other defenses when they attempt to land? They're large, highly visible objects that have to decelerate slowly enough not to kill the troops on board, making them good targets. $\endgroup$ Aug 1 at 3:45
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    $\begingroup$ If you drop paratroopers right onto the enemy positions, they would be at a disadvantage and many end up dead. If you drop them outside enemy positions, they would have to assemble, regroup and attack just like ground troops do. $\endgroup$
    – Alexander
    Aug 1 at 8:50
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    $\begingroup$ Why do the current day military “superpowers” have ground armies when it’s all about air superiority? It’s simply that carpet bombing is not always the best solution. $\endgroup$
    – Michael
    Aug 1 at 10:26
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Why are you making war in the first place? Your reasons dictate your methods, because the costs aren't always acceptable. Usually, though, you're not fighting a war of extermination: I'm going to be running on the assumption that you're referring predominantly to wars of conquest, with somewhat more nuanced objectives than "kill them all".

One reason for ground forces is that you need them to, well, hold ground. You can't effectively govern a city entirely from orbit, even if you get them to surrender somehow. You need to enforce curfews and martial law and so on, whatever policies are necessary to maintain order, and you need boots on the ground to do that. If you stick to just orbital surveillance, you are guaranteed to miss most dissident action, subterfuge, etc.; in other words, you don't actually control the territory at all.

Reason number two: in a war of conquest, you want to acquire something the other side has, typically territory and/or resources. Those wide fertile fields of Planet Farmerville aren't going to grow any crops after they've been reduced to wastelands by bombardment. Those valuable mines on Planet Minertown, full of fragile unobtanium that's worth twenty million credits a kilogram, collapsed by your nuclear bombs? Great job denying the enemy that resource, but you're cutting off your nose to spite your face at that point, since you've now destroyed your reason for trying to take the planet over in the first place.

Reason three has to do with precision. I already touched on this with how bombardment will tend to pulverize the very resources you're trying to claim, but it goes beyond that. If your standard approach to warfare is to blast apart all opposition from orbit, you will be killing a lot of civilians along with the soldiers. You're going to turn the survivors into a massive guerrilla force: even if you win, the ruins left behind aren't going to be anywhere near profitable or useful enough to justify the cost in blood and gold. That sort of massacre is also going to turn others against you in the political arena; it's going to escalate the war, and see similar retribution against your own people. This is basically why the laws of war were invented: they help keep wars from spiraling into the sort of Pyrrhic disasters where both sides lose.

I also see you wondering why beachheads are necessary as a specific example. This one is a fairly simple problem in concept, even if the reality is something military professionals are forever balancing: numbers and cost. Why didn't the Allies just deploy an army entirely of paratroopers in the D-Day invasion and bypass the bloody amphibious beach assaults altogether? Drop pod tactics (and rigors: riding in one of those is going to be much harsher on the troops than a transport) are for elite soldiers. You can't feasibly have your entire army trained to the level of special forces: the bar is a lot higher than for regular troops, which reduces your total army size, not to mention the additional cost of training and equipment for those specialists. Elite soldiers have their place (drop pods would be excellent for disrupting lines), but they can only do so much if they're outnumbered six to one by the other side going for regular troops instead.

Also, troop transports are going to be a lot cheaper for troop deployment than drop pods. Using transports will net you far more troops on the ground overall than drop pods; you sacrifice some flexibility in deployment, but both have their roles. Using dropships or shuttles rather than drop pods means you don't need as many vessels dedicated to transportation of forces (or as much space on your existing vessels, if they're all multi-purpose warships). Keeping costs down means more warships or ground forces can be built/trained and outfitted, which also helps raise your numbers.

As for the beachheads specifically, drop pods and similar tactics are not practical for resupply purposes: they're hideously inefficient for providing extra ammunition or other war materiel and don't offer a means to retrieve the wounded, reposition soldiers if a local retreat is needed, etc., so you need those transports regardless. However, transports are usually underarmed and underprotected (hybrid transport/combat-armed shuttles are a lot more expensive and typically less effective than specialized vessels), so they need safe places to land: beachheads. Major targets are probably well guarded by anti-aircraft fire, not all of which can feasibly be removed by targeted orbital strikes. Conventional two-dimensional war maps with front lines are a stretch, but on densely populated planets with numerous well-defended points of interest and a strong defense force, they might happen, especially if there are any ground-based defenses powerful enough to reach orbit and thus limit or deny any of your forces in the air or space near those areas.

Conclusion: You need ground forces to take control of territory, to capture its resources and put them to your own use. Bombing might be the most efficient and least costly means of killing, but warfare (usually) has some other objective than genocide or mass slaughter, and the collateral damage will wreck the value of whatever you've conquered.

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  • $\begingroup$ Regarding Point 3. There is technically no reason an orbital strike has to automatically 'pulverize' a specific target or area. It certainly can but it doesn't have to. The attacker has the option of scaling the force & or type of weapon deployed to achieve their desired outcome. If a population won't surrender (& there are no legal or moral impediments) your choices include using WMD that 'purge' selected targets of rebel civilians while minimizing damage to infrastructure. You also have the luxury selecting any point on the planet as your LZ - & then letting enemy come to you. If they dare. $\endgroup$
    – Mon
    Aug 2 at 1:09
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    $\begingroup$ @Palarran Sure; but you could also drop a very much smaller metal rod which would destroy one house and leave the next-door house untouched. We can assume given the tech level that guidance systems are essentially perfect. So you could drop a thousand tiny metal rods to eliminate a barracks up to the boundary fence, without touching anything outside the fence. $\endgroup$
    – Graham
    Aug 2 at 2:05
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    $\begingroup$ Or many different types of beam weapon, or robotic drones that skim the upper atmosphere and deploy sub munitions, or EMP devices, or bio weapons/gasses (sub-lethal or not) or swarms of relatively compact hunter/killer bots parachuted to the ground in pods programmed to 'swarm' selected targets', or you 'nuke the site from orbit' a couple of times and watch the clean up op from on high , or, or or..... $\endgroup$
    – Mon
    Aug 2 at 2:17
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    $\begingroup$ All the talk of precision strikes doesn't remove the human factor. If the local population is determined to resist to the very end or at least refuse to work for the invader, then the invader has to ship in their own work force. You now have to put rubber boots on the ground $\endgroup$ Aug 2 at 3:57
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    $\begingroup$ @Palarran You don't need fuel, you just need enough batteries to drive steering fins for a few minutes. We're talking guided bomb, not missile. Also at high decent speeds, crosswinds won't have as much effect. $\endgroup$
    – Graham
    Aug 2 at 11:13
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Why can't we establish a beachhead behind enemy lines?

If I'm understanding your real intention, it's to establish why you need to fight ground wars with large conventional forces instead of just bypassing all that and directly assaulting the enemy at each individual site where there is any resistance. Special forces teams inserting everywhere to disrupt the enemy everywhere screws up your vision of warfare. So lets think of some reasons why that would be.

  • ships aren't traveling through regular space, so troops aren't really dropping: While space in your universe is similar to space in ours, it's not REALLY in the same universe. To go faster than light, you bypass the physical universe and enter an alternate "hyperspace" (aether, ethereal plane, whatever). Opening a hole into hyperspace is really hard. A team of wizards push themselves near to death to make a portal into hyperspace and vice versa. And while ships in hyperspace can "see" the real world, they must dock at special ports ("space stations") where the portals to the real world are located. Thus, once established, only the largest worlds would have more than one portal, and all portals from a world would likely go to the same space station. You REALLY want your portals in secure locations, as you lose the whole world if you lose even one portal (and thus your station).
  • Ships are repelled by life, land, hostility, etc.: Something on the destination world makes it very difficult for your ships to land in most locations. Perhaps life repels vessels, so ships can fly through space easily, but a jungle is like a force field. Ships can only land in deserts, wastelands, the arctic, or POSSIBLY during the winter. Or perhaps water allows landing, but land repels ships. Maybe the negative emotions of enemies towards your ships acts repulsively, so unsuspecting natives allow ships to land, but those actively opposed to them unconsciously repel attempts to land. Established beachheads in friendly (literally) ports are the only places vessels can easily go.
  • Ships are too expensive to land anywhere they can be put at risk: It requires years of GDP to build a stardrive. So much so, all the ships are huge things to take advantage of the size per drive. Only ships can land, and attempts to drop men down a gravity well are either one-way (bye-bye dropped troops), a disastrous risk of men (not more dead paratroopers!) or risking your insanely expensive and huge vessels.
  • What goes down doesn't necessarily go back up: A variation on the stardrive reasoning, it's really hard to get out of the planetary gravity well. Large, complex launch facilities are needed to lift most ships off a planet. So while you can drop your troops down behind enemy lines, it's like parachuting men during WW2 - you can't just land a chopper to pick them up, but have to construct a launch facility (the equivalent of a runway/airport) or leave them where they fall. This GREATLY limits the tactical use of dropping valuable special forces behind enemy lines, while still preserving it as an option or allowing a few specialty ships (adventurers...?) to carry out landings.
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How else does one hold ground? There has to be some kind of force to hold onto the important stuff - it doesn't necessarily need to be humans, it can be mass-produced robots, or golems, or what have you - but a small team of elite marines cannot hold terrain, and neither can an orbital-bombardment craft.

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    $\begingroup$ You still have though that the enemy has a 'God's eye view' ' of the battle space from up on high. And while its relatively easy for him to send birthday presents 'down' its much harder for the defender to return the complement. So troops 'holding ground' would have to be highly (think extremely) mobile to avoid being hammered. And of course while your front line troops might be able to manage it all the support staff need fixed locations to work from. And the enemy? He's got an entire planet to choose from as an LZ when (and where) he decides the time is right. $\endgroup$
    – Mon
    Aug 1 at 4:02
  • $\begingroup$ @Mon Sure, but there is literally no other way to hold ground. They might scuttle into bunkers whenever a fleet arrives, but they need to be there to hold ground. $\endgroup$
    – KEY_ABRADE
    Aug 1 at 4:44
  • $\begingroup$ not true. if you control the orbits, you also control the ground, once you've smashed all the defensive installations. $\endgroup$
    – ths
    Aug 1 at 10:10
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The problem (for the defender) is that once the orbital space above your planet has been conquered it becomes next to impossible for you to concentrate any kind of conventional military force (think tanks, guns, troop formations, etc.) for the purpose of defending key portions of the planet without creating a target for the ships in orbit. All sorts of nasty precision weapons can be dropped on your HQ's, supply points, and troop formations.

This forces you to either;

A) Disburse in small packets across the planet, which is not a formula for victory, merely a delaying tactic.

B) Hide in plain sight - concentrate in and around major population centres or other high value assets like space ports/power plants hi-tech facilities that the enemy doesn't want to damage. Again though, that depends on how sensitive your foe is to inflicting civilian casualties and how confident you are of relief arriving or some other intervention occurring. Or how fanatical you are. Mind you if they won't risk bombarding the cities etc you occupy then you get what you wanted i.e. they have to come down and fight a 'conventional' battle.

C) Retreat to hardened fortresses deep underground or protected by 'force fields' etc. and defended by heavy weapons. Then try and force the enemy to either dig you out or use WMD that they might not want to deploy because they want the planet in a livable state for themselves.

None of these are very palatable choices though.

Your best ground defense might not be conventional forces trained to fight pitched battles at all. Instead train and deploy as large a force of special forces/commando type troops as you can afford. These troops are specifically trained to work in 'cells' and are given the caches of equipment sufficient for them to run a very damaging insurgency operation (your words)/terrorist campaign (your enemies' words) against occupying forces and just try to wear them out. Think Northern Ireland or Afghanistan.

Again though any such operation's success will also depend on how committed the civilian population is to ejecting the occupiers. Without their ongoing passive/tacit support it wouldn't work against a really determined/ruthless opponent.

EDIT: Ultimately though? If the enemy is absolutely determined to hold the planet no matter what the cost you can't force them out unless or until some kind of relief force arrives to recapture the low orbits. At which point the invader is now the one with all the unpalatable options.

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    $\begingroup$ This. Consider how it goes in the Honor Harrington universe--once you control the orbit of a planet you can demand it's surrender. You may only fire upon a planet if it refuses to surrender--but normally it will surrender at that point. $\endgroup$ Aug 1 at 4:04
  • $\begingroup$ @LorenPechtel, you beat me to it. Also IIUC, a planet that refuses to surrender forfeits Eridani Edict protection, which is to say that if they don't surrender, indiscriminate bombing becomes "acceptable". Which is a good recipe for the majority of the population to turn on the idiots refusing to surrender. OTOH, planets of fanatics happen, especially if religion is involved or if there's some other reason for most of the population to hate and/or fear the attackers. Honorverse has multiple examples of why ground forces are still needed. $\endgroup$
    – Matthew
    Aug 1 at 12:49
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    $\begingroup$ All of which tends to prove my main point. There's no realistic/easy way to 'defend' a planet with convention land forces once orbital space has been occupied by an enemy. All the options I summarized are 'bad' from a conventional strategic perspective (much less a humanitarian one). Even the asymmetric approach I outlined, which might (I stress might) work in the long term also guarantees many months/years of hardship for the civilian population. So cardinal rule? DON'T LOSE THE ORBITS. $\endgroup$
    – Mon
    Aug 1 at 13:07
  • $\begingroup$ @Matthew Honorverse has very little in the way of use of ground troops--although there are several cases of boarding operations against pirates. Off the top of my head the only marine action against a nation that comes to mind is when they were rescuing the crew of that freighter from the Sollies--and the only reason for that is they were trying to keep the death toll down. Legally they could have pounded the planet at that point. $\endgroup$ Aug 1 at 22:49
  • $\begingroup$ @LorenPechtel, have you read On Basilisk Station? Or the Shadow spin-off series? Masada is outright occupied, and Kornati has an ongoing problem with the crackpot NRP. Then there's, what, three civil wars on Haven? (Although those involved both ground and atmospheric forces.) Plus there are non-combat appearances such as disaster S&R, and only some of those examples are marines. All, however, are largely offensive; mind Mon's comment! $\endgroup$
    – Matthew
    Aug 2 at 13:38
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Maybe the defenders have effective surface-to-orbit weaponry. With STO weapons defending key military objectives, the attackers can't just sit in orbit and bomb everything. Ships big enough to do effective orbital bombardment get gunned down by the STO batteries. But troop transports and drop pods can be small and nimble enough to avoid STO fire. So ground forces are deployed first to take out enough of the STO batteries to clear the skies for the capital ships. Kinda like what happened at Reach in the Halo universe.

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There are so many possible answers to this. Some obvious ones include:

  1. Dropping troops directly onto enemy positions may tend to be difficult or costly, especially if the enemy are hard to locate and bombard from orbit first. It may be difficult or expensive to drop large numbers of troops in one place, while it may be easy and cheap to assemble effective fighting forces whose firepower can cover the places you would drop troops. It may also be dangerous to drop troops, because they can be observed and fired on from the ground both in their ships and while dropping.

  2. The ground, terrain, and curvature of the planet may tend to be usable to provide cover and concealment that is not present in space, low orbit, or the air, which may mean if you deploy many forces above-ground, they tend to be much easier targets than if they are on the surface, hiding, camouflaged, undergound, underwater, under clouds, and/or hiding in or near places/people/infrastructure that the enemy does not want to destroy.

  3. In order to actually control areas on the planet, it may require a large number of troops, typically because it is hard to distinguish hostiles from civilians without enough people there with them.

  4. Supply issues. Most technological weapons don't have infinite shots. Most troops require food, medicine, and places to sleep. Most technology requires fuel, supplies, spare parts, and maintenance equipment. Most armies require communication. All of that is much easier to provide, if you have full control of territory. That is what having front lines versus rear areas tends to largely be about.

  5. Another thing front lines versus rear areas is largely about, is tactics. Reconnaissance, concentration of force, being able to fight in a single direction rather than caught in crossfire, and other tactical considerations argue for wanting a strong formation which can keep the enemy in front of it, and that can count on little risk of attack from the sides or rear. Dropping your forces right onto the enemy is a great way to have chaos and be surrounded by foes.

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We're in the "space age" now. Suppose space technology is just the same, and interplanetary portals just go straight to the enemy planet - no spaceships exist. (We have 'space craft' in RL, to be clear.)

Also the portals can only be roughly targeted, e.g. it opens on the enemy planet we found but it's not controllable where. You might end up at the poles, over the sea, on reasonable land... you'd need a way to scout new planets you find of course. Send a stealth recon plane through first, or similar.

So the answer would be, no attacking drops of space marines because we can't get to space above enemy planets to begin with. (Maybe some enemy civs would have that tech of course. Better hope they don't find us.)

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