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Aliens are invading earth! Pouring from portals hidden deep in the earth for millennia, they are sweeping aside the militaries of earth with contempt. Modern infantry rifles, even loaded with armor piercing ammunition cannot penetrate their fully body armor suits. So far, the only small arm to damage an alien soldier has been a large-caliber anti-materiel rifle. Tank rounds and heavy artillery are effective, but without infantry support they are easily overrun and destroyed.

Alien soldiers are equipped with conventional firearms that are unable to penetrate their own armor. (This is believed to be a chillingly pragmatic choice to prevent us from copying their technology.) These weapons are roughly equivalent to a M60 LMG and are used as assault rifles by the physically stronger alien infantry. They do, however, deploy a highly effective anti-armor missile system.

The alien portals are scattered across all of earth's continents, and no one is sure that all of them have been identified. Several of them are sited underneath major population centers. Alien force strengths are difficult to estimate, but human forces outnumber them by 3 to 1 on average. Alien troops receive supply through their portals, but have been quartering troops in human structures.

Alien armor is, from the few samples recovered, an advanced ceramic plating. Ballistics tests and field reports put it on par with the armor on a modern military armored car, but much lighter and more mobile.

The question is this: What can humanity equip its infantry soldiers with to counter this enemy?

Assume:

  • Access to all the military technology of today's earth
  • The weapon should be either already broadly available or easily mass-produced. The forces of earth need access to it yesterday.
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  • $\begingroup$ @Cragor Question updated with more info. $\endgroup$
    – Danny Reagan
    Commented Nov 24, 2014 at 13:48
  • $\begingroup$ Do they have any vehicles (air- land or sea-based)? $\endgroup$
    – Philipp
    Commented Nov 24, 2014 at 15:07
  • $\begingroup$ @Philipp No large alien vehicles have been seen, but they do make use of a spider-like version of this: time.com/2987608/us-military-robotic-mule to carry supplies and munitions. $\endgroup$
    – Danny Reagan
    Commented Nov 24, 2014 at 15:09
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    $\begingroup$ If aliens used rifles unable to penetrate their own armor, I would think humanity would pour a lot of resources in retro-engineering or adapting pieces of armor for human use. But then I guess the aliens would start using alien Barret M82's? $\endgroup$
    – EagleV_Attnam
    Commented Nov 24, 2014 at 16:49
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    $\begingroup$ I personally would equip myself with a hand held sign stating my desire to switch sides and pointing at my uselfulness at managing the scum of the soon-to-be slave work force that was once called mankind. $\endgroup$
    – SJuan76
    Commented Nov 24, 2014 at 18:58

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The best bet isn't infantry but our air force and vehicles.

You only mentioned alien infantry, but no sea- air- or land vehicles. That would make the invaders quite immobile after they exited their portals. It would be easy for a motorized army to outmaneuver them as soon as they left the city and are in the field.

Many battle tanks are equipped with machine guns of larger caliber. The US M1 Abrams battletank, for example, has a cal.50 machine gun which uses the same caliber ammunition as anti material rifles, so they would be highly effective against the invaders. There are also armored troop transports equipped with even larger automatic machine guns like the M2 Bradley which has a 25mm machine gun with a fire rate of 200 rounds per minute. Many other armies have similar vehicles.

Another huge disadvantage of the aliens would be that humanity has complete air supremacy. High-altitude surveillance planes and drones can monitor all troop movements and tactical and strategical bombers can attack them. Modern bombers equipped with smart-bombs can accurately hit a target from an altitude of several km. Most anti-infantry bombs which use shrapnel as their main method of killing targets would not be as effective as against unarmored human infantry, but high explosive bombs would still be pretty effective. Incendiary ammunition might also work.

And when all fails, we still have our huge stockpile of nuclear weapons. When a city is lost and it is certain that no remaining humans will be left alive, obliterating it with a nuclear strike before the invaders disperse might be a hard but pragmatic decision.

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  • $\begingroup$ Good points. Although the aliens do have, as mentioned, a very effective infantry-portable anti-armor weapon. Even human shoulder-launched AT missiles are deadly to tanks, requiring infantry support to counter. Air power would remain effective, as long as air bases could be protected. $\endgroup$
    – Danny Reagan
    Commented Nov 24, 2014 at 15:31
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    $\begingroup$ @RobertBoettcher The effect of a single nuclear explosion on the global biosphere is often overestimated. Keep in mind that during the cold war, several hundred above ground nuclear weapon tests were performed. $\endgroup$
    – Philipp
    Commented Nov 24, 2014 at 15:56
  • $\begingroup$ Obliterating a lost city with a nuclear strike will only serve to hasten the demise of the human resistance. Its only use might be the threat to wipe out the entire biosphere when the attack is not blown off. Supposing the Aliens actually want to take posession of the planet, spoiling the spoils for them might be a reasonable deterrence. They might actually be able to look through those doomsday policies however... $\endgroup$
    – Robert Boettcher
    Commented Nov 24, 2014 at 15:57
  • $\begingroup$ @Philipp, the Aliens would not take just one city, but use their momentum to gain an advantage. If on the other hand Nuclear Weapons are used to deny that advantage, its immediate results might be just what was intended. The soot from the burning city, increasing the albedo of the planet and reducing the sunlight that reaches its ground, causing massive agricultural and climate disturbance, makes that rather self-harming than anything else. $\endgroup$
    – Robert Boettcher
    Commented Nov 24, 2014 at 15:58
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    $\begingroup$ @DannyReagan : "shoulder-launched AT missiles are deadly to tanks" - no, they aren't. A shoulder-launched missile can indeed destroy an armored patrol vehicle but will not do more than scratch damage against a modern main battle tank. $\endgroup$
    – vsz
    Commented Nov 25, 2014 at 7:10
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The thermobaric devices can be incredible. If you could get one to go off in a reasonably enclosed area, with a large number of troops, you would basically cook them all in their armor.

Basically they usually have a smaller explosions to accelerate and spread a combustible material out and mix with the local Oxygen, then it ignites causing a huge explosion, often with an incredible pressure wave, lots of heat and completely using up the local oxygen supply. Some of these have near nuclear level blasts, without all the fallout and other dangers from nuclear.

ETA and as Robert Boettcher pointed out in the comments, they are much easier to manufacture than Nukes too.

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    $\begingroup$ If those can be brought out over the portals, they would pretty much prevent any reinforcements from reaching the battlezones. As they are conventional explosives, the only limitation of their use is their constant manufacture and deployment. $\endgroup$
    – Robert Boettcher
    Commented Nov 24, 2014 at 15:48
  • $\begingroup$ If you use liquid oxygen instead of atmospheric oxygen they get even more effective, with yields on-par with tactical nukes, and manufacturing complexity such that manufacturing speed is limited only by available electricity. When Germany tested a few in WWII we thought they'd beaten the Allies to the bomb. $\endgroup$
    – Perkins
    Commented Oct 24, 2016 at 23:50
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BOOM.

If I was going up against anything with this level of armour, I'd want to be wielding something big, something that just will not care how much armour there is. What options does this give us? Anything that can take down a tank (or any other kind of armoured vehicle) will probably be pretty effective, except that we're dealing with a smaller target.

Concussive forces, and intense heat, are going to be our friends; the modern rocket-propelled grenade (RPG), or any other grenade-launching weapon, will be very useful. I suspect any grenade will do, but something with a heavy charge or fire especially, unless their armour resists heat somehow. There are also the 1900s-era gas grenades, if these invaders are vulnerable to poison at all. You could also consider mines! There are some that are even magnetic, if the armour is iron or steel.

Flamethrowers could be terrifying for any force, albeit usually illegal (maybe the governments will make an exception?) as it will turn their armour into an oven. There are several high-tech weapons employed by some modern military forces that could also work; I've heard of one that essentially launches a charge that penetrates a wall and expels molten metal that cooks the interior, but I can't find a source for that.

TL;DR: Anything that produces fire, explosions, or that could work against a modern tank/armoured vehicle.

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  • $\begingroup$ Flamethrowers sound like a good call, but I don't know how effective RPGs would be against infantry, no matter how chunky they are. $\endgroup$
    – Danny Reagan
    Commented Nov 24, 2014 at 14:10
  • $\begingroup$ Yeah, I agree with you there; kinda why I didn't suggest explicitly any of the heavier weapons often used. Biggest thing with them is that you aim near them and hope the blast does some damage. Standing next to an RPG when it blows up would probably kill us, so it might at least hurt your aliens. They're also ridiculously common weapons it seems, and fairly portable for infantry. Think of them as a grenade-delivery system more than an anti-armour rocket. $\endgroup$
    – Crabgor
    Commented Nov 24, 2014 at 14:12
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    $\begingroup$ A few points on the flamethrowers: They're not banned (except against civvies or soldiers within high concentrations of civvies); they have a very short effective range (especially relative to an enemy that can carry and wield an M60 as if it were an M4!); the pair of big heavy tanks on your back don't actually carry enough fuel for more than few short bursts, and have a tendency to fail catastrophically when struck by enemy fire. Primarily due to their ineffectiveness, the US DoD (unilaterally) stopped using them post-Vietnam. $\endgroup$
    – Kromey
    Commented Nov 24, 2014 at 17:47
  • $\begingroup$ No, RPG's with fiery explosions is a kind of a Hollywood idea. There's an episode of Mythbusters showing what an RPG looks like, and the explosion is very shaped (to punch through armour). They'd be highly effective against these aliens I suspect. Napalm is an ignited gel that sticks to you and then continues to burn for some time. Ceramic armour (as mentioned in the question) is quite a good insulator of heat. But I don't think good enough to prevent being cooked inside their armour. $\endgroup$
    – Shaun K
    Commented Oct 14, 2016 at 0:17
  • $\begingroup$ Anything with significant concussion could be a good weapon to use. With concussion you don't necessarily need to get through the armor to kill the trooper inside. $\endgroup$
    – Jim2B
    Commented Apr 15, 2017 at 2:03
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Fragmentation Grenades

Since Bullets are small, and their kinetic energy obviously absorbed by the material of the Aliens Armour, an approach might be conducted into testing the viability of explosive distribution of shrapnel in order to open up vulnerabilities in their armouring, which might then become attackable by small arms fire.

Since Fragmentation Grenades are, in some Units like the Marines as a Standard Operating Procedure, used first upon approaching targets, their ready availability would be assured.

http://en.wikipedia.org/wiki/M67_grenade

Grenade Launchers

Alternatively, one might try to detonate a grenade shot from a M79 at the Invaders.

The idea is again that explosively released shrapnel, with the millionfold expansion from the original volume of its explosive material, would have a better piercing effect than the kinetic drive produced by the gases released upon firing a firearm.

Since every regular infantry squad is to be equipped with a grenade launcher to aid the breakup of enemy positions in hiding, or where the machine gun cant fire, its ready availability would be assumed.

Grenade Launchers

Chem Weapons

As Aliens are not humans, the Conventions against using Nuclear, Biologic or Chemical Weapons would not apply to them.

However, the after effects of Nuclear Weaponry (devastating to the planet, but surely for them as well), and biologic warfare (devastating to our lifeform, but perhaps not to theirs) would rule out such measures.

However, Chlorine for example is readily available and if brought out in sufficient concentration might be able to corrode or cauterize their armour, making it more vulnerable to attack with the explosive methods mentioned before.

If artillery can bring out barrages of Chlorine gas over the portals it could hinder the Alien Troop Movement by mentioned corrosion or at least by reducing visibility of the terrain.

For this purpose any corrosive chemical compound, widely used in Industrial applications, could be weaponized by filling it in artillery shells, or by even simply having soldiers equipped with spraying equipment unload the chemical on the Alien occupied by (effectless) firearm teams with a simple hose.

Flamethrowers

While not widely used since WWII, flaming agents might be considered as the thermal stability of the Alien Armour is not yet known, but can be safely assumed to not be arbitrarily high.

Even highly efficient nano-carbon structures that might produce before mentioned protection against fireweapons would probably crumble to dust when exposed to high enough temperatures due to burning flaming agent stuck to it.

As Napalm can be relatively easy manufactured from Gasoline and Acid, it would be an option that can be considered together with the Chemical Components when selecting a mobile delivery system, as it would at best have to handle most of those agents in one set of equipment.

The detriment is that flamethrowers are not featured anymore in the current arsenals. However, a delivery system that would be able to deliver the (non) burning agent onto the Antagonist Alien could be as simple as a firehose pumping the agent onto the target.

Further suggestions

The list could be continued with fuel-air bombs or even tactical nuclear artillery shells, but since the nature of the scenario appears to be pitched infantry battles, the options end here.

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  • $\begingroup$ +1 For the acid weapons. Would the alien's armor being made from ceramics affect acid's effects? In addition, would shrapnel really penetrate better than 7.62 AP? $\endgroup$
    – Danny Reagan
    Commented Nov 24, 2014 at 15:43
  • $\begingroup$ Even though few armies still have regular infantry flamethrowers, it is a weapon which is very easy to improvise. Any portable device which sprays liquid can be turned into a flamethrower by filling it with a flammable liquid and duct-taping a zippo to its nozzle. Such devices are mass-produced for lots of civilian purposes (gardening equipment, high-pressure cleaners, even water pistols for children). $\endgroup$
    – Philipp
    Commented Nov 24, 2014 at 15:47
  • $\begingroup$ I'm not sure there is a way to compare ballistics with explosives, but I believe that the amount of explosive found in a firearm round is smaller by a power of magnitude from the amount of explosive in a grenade. The idea that flying objects at a high enough velocity can pierce the armour is still rather conventional, but the corner point of the attack strategy. In this regard, you produce more and faster flying stuff with a grenade than with a single bullet. So I guess shrapnell will do better. $\endgroup$
    – Robert Boettcher
    Commented Nov 24, 2014 at 15:54
  • $\begingroup$ Air crews in WWII wore flak jackets to protect against shrapnel, but those same jackets didn't do much against bullets. This suggests that shrapnel is easier to defend against than bullets. $\endgroup$
    – Danny Reagan
    Commented Nov 24, 2014 at 15:56
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Love

Okay, it sounds cheesy, but hear me out. What you described suggests a race which would view us as nothing but unruly furniture, so it is very likely that they would be coarse when modeling our behavior. This means that, in the heat of battle, subtle actions and choices that we make would be completely overlooked by the enemy. That would mean there is an entire set of things we can do which are virtually invisible to the enemy due to their warlike nature.

Rather than going for a military victory, I would begin going for a cultural victory. Take advantage of these subtleties to infect them with the most positive sides of our culture we can find (I, personally, find the "good stuff" of culture to always be subtle and sublime). Make is to that they can take out planet physically, but we take their culture right out from underneath them such that they can no longer distinguish between us and them.

There are several words we use in modern day life which have the subtlety needed. The two which seem most likely to inspire an army to the crazy sort of opposition are:

  • Love - consider the wonderful picture of a '60s hippie putting a flower in the barrel of an opposing gun. They'd never see it coming. Also consider that "keep your friends close and your enemies closer" evolved from a teaching about Love.
  • Stillness - The Eastern cultures would be able to use their meditative cultures to observe ways to make our army appear to be completely still, while they are actively moving in ways to teach the aliens why we don't need to be eradicated. There are several martial arts disciples which focus on just being still and letting the opponent do the work.

Remember, the effects of these concepts would be magnified by the fact that Aliens do not have an entire life and upbringing experiencing the effects of fake love (a.k.a. they never survived puberty at an Earth-dwelling High School), so those who have to "fake it" would find reasonable success.

Even if the "carrot" approach of cultural victory fails, it is highly likely that we would develop martial arts to take the "stick" approach to take advantage of their natural need to underestimate us (why waste brainpower predicting us when you know you can smoosh us like a bug).

When the brute force approach fails, we eventually learn to be more subtle with our approach.

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    $\begingroup$ I just had comment under my answer, so I am going to repeat it here for you in other words: 1) What if the ultimate goal of aliens is to kill us all? (for whatever reason) 2) What if the aliens perceive us as horrible bugs? Would you not kill dangerous insect if it remain still? $\endgroup$
    – Pavel Janicek
    Commented Nov 24, 2014 at 16:37
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    $\begingroup$ I wasn't sure if the "stillness" line was going to get me in trouble or not. The martial arts that use "stillness" are not actually still, but rather rely on moving in ways which are terribly hard to see coming, often using your own movement against you like Aikido does. The result is a "dangerous insect" that appears to be still and calm, but seems to dodge every blow you send its way as though he was moving at unbelievable speeds. They're a really neat class of martial arts, which are totally non-obvious if you've never seen them in action! $\endgroup$
    – Cort Ammon
    Commented Nov 24, 2014 at 16:42
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    $\begingroup$ @PavelJanicek I agree. It's a nice idea, depending on the ultimate goal of the invaders, it may be moot. However, it is a valid option, even if it only leads to backstabbing when they join us in peace. But, if we can't go for a domination victory, or a cultural victory, there's really only one option: TO SPACE. (if anyone gets the reference...) $\endgroup$
    – Crabgor
    Commented Nov 24, 2014 at 16:44
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    $\begingroup$ Yeah, the ultimate goal would be an important question when trying to approach the aliens with the one I used. However, I do argue that it doesn't matter what their ultimate goal was at the start of the war, what matters is what their ultimate goal is at the end of the war. If we can subtly shift their goals without them noticing, then what difference is it to them? Many a war has been won by shifting the enemy's goals to not include your extermination. $\endgroup$
    – Cort Ammon
    Commented Nov 24, 2014 at 16:50
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Disclaimer: I am by no means an expert in biology or war tactics, feel free to point out any gaping plot holes I have missed.

Biological Warfare

As used in 'The War of the Worlds' by H. G. Wells

To Quote:

And scattered about it, some in their overturned war-machines, some in the now rigid handling-machines, and a dozen of them stark and silent and laid in a row, were the Martians--dead!--slain by the putrefactive and disease bacteria against which their systems were unprepared; slain as the red weed was being slain; slain, after all man's devices had failed, by the humblest things that God, in his wisdom, has put upon this earth.

Either weapons could be tested by trial-and-error in the field (e.g. Mustard Gas), alternatively, if a few of the Aliens could be killed with the rifles mentioned, and the bodies could be captured for examination, then human researchers could discover any weaknesses of the Aliens' immune system (or other parts of their biology) and use this to counter them.

If the Aliens are quartering their troops in human quarters, then any biological weapons that could not be introduced on the field (if the Aliens has gas masks or similar) could be planted in conveniently abandoned buildings (such as a genetically-engineered virus), with the hope that the Aliens took the bait and stayed in the infected building. This would be when the Aliens are at their most vulnerable since they may well be out of their armor, showering, sleeping etc and would be the most likely time for infection to occur.

Providing the virus introduced was infectious enough, if the Aliens weren't very careful with their quarantining it could soon spread to the rest of the troops (and potentially back through the portals).

If the Aliens' immune systems are different enough from humans' it may be possible to use a virus that is fatal to the Aliens but harmless to humans, such as a cold virus.

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    $\begingroup$ The straight dope did a great article on how effective a virus or bacteria would be against non terran life straightdope.com/columns/read/2606/… the conclusion was.... almost certainly zero $\endgroup$
    – Mikey Mouse
    Commented Nov 26, 2014 at 11:07
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It may be a bit unconventional, but things like Molotov cocktails and caustic agents would be very effective, especially since they are so heavily outnumbered and heavily armored. Heavy armor (no matter how advanced) becomes a liability if anything can get next to the skin or clog the air inlets. Then it's a matter of leave the armor on and suffer/die or remove it and get shot/die. Without numbers and/or vehicles it'll be a very short war.

Just keep them busy enough to cut off their supply lines and eventually their conventional weapons would run out as well as their food.

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    $\begingroup$ +1 to this. Also phosphorous grenades and ammo, napalm and its improvements, microwaves ... Anything that raises the temperature inside the alien suit above the threshold of the alien. Any cooling requires energy and in a small combat suit the energy is finite. $\endgroup$
    – Juha
    Commented Nov 24, 2014 at 20:15
  • $\begingroup$ Honestly, if it gets to that point, you can just perpetually spray them with fire extinguishers (assuming they need oxygen to survive). If you keep it up long enough, there will be no air left to breathe around them, and they suffocate, unless they have backup oxygen tanks (a liability, as pressurized oxygen is highly explosive). $\endgroup$
    – DevilApple227
    Commented Sep 1, 2016 at 16:42
  • $\begingroup$ Going big with this idea: incendiary bombing a region to deprive it of oxygen ala Dresden Germany during WWII. $\endgroup$
    – Jim2B
    Commented Apr 15, 2017 at 2:04
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So, we have aliens that are the equivalent of light tanks attacking us. What do we do to light tanks? SLAP them around with the slighly larger 7.62 caliber found in the notoriously reliable and cheap AK47? If normal .50 BMG bullets can kill them, it isn't too hard to imagine a smaller round with HEIAP or SLAP doing damage after a few shots.

Not working? Call in air support! I'll imagine even the Air Force won't be as keen to scrap the A-10 Thunderbolt during an alien invasion. You don't have to carry your machine guns any more you know. The aliens will probably want to start stocking up on anti air missiles now...

Now, we've found most of the gates, so let's just destroy them quickly and see where they get their supplies from then. Cruise missiles are a relatively cheap method of medium range explosive delivery.

And I'm sure you've heard before, bad things happen when you start doing science in a pressure cooker. Putting it on a cruise missle might not be the easiest thing to do, but it would surely bring a lot of carnage.

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The enemy - assumption

When you consider that an alien race is invading earth, then they probably have already done it before. They also traveled a long distance and unless they can gap long distances in the blink of an eye, they are likely to be something like swarming locusts on a raid that need to be able to adjust to a lot of different scenarios. If they would be able to gap possibly hundreds of lightyears in no time at all, then they are that advanced that the only chance we have is to hide and hope that we don't get distinguished.

Enemy suite

And that would bring an amount to difficulties to us as well as to the technology that they need to fight different scenarios and enemies:

  • High vs. low pressure planets
  • High vs. low gravity
  • Thick vs. thin atmosphere
  • Different types of atmospheres (gas types)

In response of a changing context, they would have to be wearing a suite that will be a combination of a space and a combat suite. That would also nicely align with normal bullets not being able to penetrate the outer shell as it would have to resist against tough conditions and high pressure. The problem for humanity would be that flame throwers, molotov cocktails or acids would not help at all as extreme temperature and corrosive fluids would be within the default space suite scenario.

Modus operandi

I would assume they do not want to have everything nuked or completely destroyed. They are here for some resource to harvest. It won't be us: Why have problematic, biological slaves that just use up additional resources that need to get transported along to the next target location when you can craft machinery to do the same and probably could clone or design every biological being to your needs?

Now we only got a limited set of scenarios left.

Fighting Machines (for completeness)

Is is very likely that we will not encounter living beings as enemies. We are already using machines for warfare, patrolling and harvesting so why should an alien race put members of their society at risk? If we fight an automated enemy, then we will likely face a decentralized system. When sending a set of machines out on a long journey then your main goals would be

  • to not fail when one part fails - redundancy
  • to repair or replace itself or parts of it - again: redundancy
  • to spread knowledge across the system quickly - shared/swarm intelligence
  • to be able to leave something back without having losses for the whole thing, making the system more flexible
  • to not need a too complex system to repair or replace parts

This means that we will mainly face one or two types of machine. This also means that the machinery will be picky on its losses. The harvesting goal can only be reached when the harvested material isn't used to replace itself until there is nothing more left. And when the missing parts can be reproduced until the next location is reached. Simple mathematics.

Fighting living beings (the actual question)

In case we would really face living beings, we probably also face the fact that there is politics in their system. And pride. I would imagine that the later one would be the only reason to fight us directly and not simply kill us all with biological warfare or by using drones. But it would also mean that the war would be much more cruel. And it would open a chance to us that we could win by fighting a guerrilla war. Losses put pressure on leaders. Especially if the amount of living resources is limited on a floating/travelling base of operations.

Mixed scenario

I personally would imagine that a race that is interested in our resources and that has traveled a long distance and is experienced and armed to fight lots of wars in different surroundings will not face us directly, but send their robot army to harvest what is left after spreading a targeted virus among us.

The only reason why they wouldn't do that is the resource can be reproduced and earth is turned into a factory where we are the workers as they can't spare to leave back machinery to keep the system up and running.

The leverage

Now that we know that we better hide than fight (which won't be an option) and that winning is mostly a game of numbers, we can try two things:

  1. Fighting the enemy directly by switching tactics from attack to attack to stay as unpredictable as possible. This will bring us high losses and we can only hope that their losses outnumber our own.
  2. The other and probably better idea might be to not attack them at all, but attack their harvesting machinery (if that isn't the same thing). This would force them to protect it as good as possible and limit their movements to the places where they harvest. Also the harvesters might be easier to destroy than their masters (or their guards).

If we stick to Nr.1 and fight them directly, then our best bet would be to build a lot of improvised explosive devices. starting with the famous pressure cooker bombs to car bombs and similar. Most "ingredients" are ready and highly available. Such surroundings would drastically decrease enemy movement and make it close to impossible to use higher technology to detect traps as those are forged out of the same things that are available in a normal household and perfectly blend in. We would probably also try to kidnap some of them, do executions (and worse) to demoralize the enemy and make every loss count double.

For more ideas, I would suggest to search for "asymmetric warfare".

Sequel

In any way, the real and most frightening problem will be that they know that we got their resources. And if they don't harvest it all, lose the (first) battle or in case the resource is reproducing, they will come back. And they will be better prepared.

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    $\begingroup$ Nice answer, but you kind of go away from the question. The question specfices that the enemies will be living. If you think the question is unclear, try commenting, so you don't waste too much effort. $\endgroup$
    – DonyorM
    Commented Nov 25, 2014 at 10:30
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    $\begingroup$ @DonyorM Fixed and extend by your comment. Thanks for pointing that out. $\endgroup$
    – kaiser
    Commented Nov 25, 2014 at 17:05
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EDIT: Thanks to comments, I need to make something clear: This answer is built around assumption, that ultimate goal of aliens is not to kill us all, but rather to use as cheap workforce for whatever they will produce on Earth

The only winning strategy to this is Partisan war and resistance movement because ultimately, you will be run over in short time and only ways to prevent this can also demolish your own land (I am referring to the nukes)

Real world example: Iraq and Afghanistan

  • The defending army is low on tech, especially comparing to today level of USA army
  • There are low possibilities to group into mass counter attack.

So why the heck has USA (and NATO in general) army such hard time bringing peace to these countries?

(Disclaimer: I am not military expert, and I care about Iraq and Afghanistan the same way as every day commoner. So please take this into account)

  • The knowledge and advantage of terrain is taken to the max
  • The knowledge and difference of language is taken to the max
  • The religious/cultural difference is taken to maximize the willingness to fight an enemy
  • The knowledge of "we look the same to them" is taken to the max
  • The feeling "us versus them" is strongly supported
  • People able and willing to sacrifice their lives in order to kill an enemy are glorified
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  • $\begingroup$ I like the Idea of Self-Sacrificing Humans. Throwing everyone at hand into suicide-missions would make the Aliens start to count their losses. But of what use are language, religion, culture and looks in a fight against an alien invader? They would be so foreign to our society of Homo Sapiens, that we wouldnt probably not even be able to make out traces of their language, let alone culture or even religion. $\endgroup$
    – Robert Boettcher
    Commented Nov 24, 2014 at 15:52
  • $\begingroup$ The whole idea of pointing out this was to pinpoint it was used (and proven to be working) in Human-Human war. So throwing some anti-alien propaganda will be really successful and self-sustainable, because in this setup, differences are much more visible than in human vs human example $\endgroup$
    – Pavel Janicek
    Commented Nov 24, 2014 at 15:54
  • $\begingroup$ Obviously the 'Alienation' towards the Aliens can be increased, to a total frenzy. $\endgroup$
    – Robert Boettcher
    Commented Nov 24, 2014 at 16:06
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    $\begingroup$ The entire premise of this post requires that the aliens care about the political implications of killing civilians. The hoops that the USA jumps through to avoid civilian casualties is the entire reason why it has been impossible to bring peace to Iraq and Afghanistan. One could legitimately argue that by jumping through all those hoops the USA is causing far greater civilian (not to mention US soldier) casualties over the long run than would occur if they just let the military do its job in the most efficient and effective means possible and get things over quickly. $\endgroup$
    – Dunk
    Commented Nov 24, 2014 at 16:24
  • $\begingroup$ No, it is built around the premise that aliens will somehow need human workforce. During World War II the Nazis did not care (that much) about life of civilian and only contact with the Resistance was toughly punished. And still there was Resistance... $\endgroup$
    – Pavel Janicek
    Commented Nov 24, 2014 at 16:28
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Browning Machine Gun

Your OP states that .50 cali rounds are effective, and that .50 cali anti-material sniper rifles have been used to limited effect. However, the original .50 cali anti-material sniper "rifle" was the M2 Browning Machine Gun or BMG for short.

The BMG was used to such good effect in the role of anti-infantry anti-material sniper that they started producing .50 cali sniper rifles.

Many of the other answers provide non-standard tactics, with or without infantry. However the fact is BMGs are used by infantry on the front line. Under the circumstances, the infantry would quickly box in the insurgences with machine gun nests if the belligerences had armor of the "quality" you describe (think Saving Private Ryan).

Later on, I would suspect that the Human forces would quickly build technicals (read BMGs mounted on pick up trucks).

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  • $\begingroup$ Do modern armies deploy HMGs of that caliber with infantry? I've only ever seen them in static positions or on vehicles. They're much too heavy to use on the move, and must be set up with a tripod to fire with any accuracy. $\endgroup$
    – Danny Reagan
    Commented Nov 24, 2014 at 18:26
  • $\begingroup$ @DannyReagan I agree that at 38kg dry weight, its going to be difficult. But the question involved an invading force. The BMG is "light" enough to box in the enemy, especially given the stakes, but not light enough for assault. Typically the HMG is deployed for support. In this case it would be used as a creeping point defence. Besides...as others have pointed out, they would only need to hold out until air support arrives. $\endgroup$
    – Aron
    Commented Nov 24, 2014 at 18:32
  • $\begingroup$ That might be an interesting followup question: "Can a modern army win a battle/war with only emplaced or static weapons?" $\endgroup$
    – Danny Reagan
    Commented Nov 24, 2014 at 18:36
  • $\begingroup$ @DannyReagan The HMG is light enough that its not static. Its man portable (barely, and usually using two man teams). But at 3-1 advantage plus the defence advantage, I think it might work...and thats discounting combined arms...The HMG team could protect that tank you said was getting overrun... $\endgroup$
    – Aron
    Commented Nov 24, 2014 at 18:40
  • $\begingroup$ Tanks have their own HMGs mounted. I fail to see how having other, slower, unarmored, HMGs following them would help. And what happens when the aliens inevitably break out the grenade launchers and mortars? (Or their equivalent, at least.) $\endgroup$
    – Danny Reagan
    Commented Nov 24, 2014 at 18:43
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Energy weapons are much closer. You all seem to forget that tech moves 10x faster in war-time then in peace. If such a war emerges, it will be a year at most before we start deploying rail guns and gauss gun. Microwaves will see an increase in use, as well as just plain old resourcefulness.

If we think short term, 50cal can be adopted for hand held use, bullpup configuration polymer/carbonfiber receiver rifle with box mags will probably be possible. It will be a return to the times of old, when you had to move a 10kg rifle that cant hit a thing, but you will have a chance.

And last but not least, they will probably unite every single war hungry human on the planet, and that is a big big big mistake.

Nukes will never be used on large scale, maybe one or two low yield ones, but are and never will be a solution. Nuke=EMP=our own equipment stops.

Logistics: -If we have full air, armored and naval dominance we will win. -We know the terrain, and thousands of years of warfare has given us the knowledge to use it. -They are depended on localized portals, nothing a few b52-ac130's cant suppress and cut out.

Some interesting things to check out(note how many of these are cold war era tech): http://en.wikipedia.org/wiki/Davy_Crockett_%28nuclear_device%29

http://en.wikipedia.org/wiki/1K17_Szhatie

http://defensereview.com/short-rifle-stock-system-srss-bulldog-762-gen-4-also-written-srss-bulldog762-gen-4-16-bullpup-m14m1a-battle-riflecarbine-with-grsc-crs-16-m4-62-tactical-scope-for-special-operations-forces-sof/

http://en.wikipedia.org/wiki/Railgun

and the unearthly, legendary and all powerful http://en.wikipedia.org/wiki/GAU-8_Avenger

There are many weapon systems never used, nor exposed to the general public, but in this situation we will see them.

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  • $\begingroup$ How about nuking the portals themselves, while keeping all the millitary far from that spot? That would create an effective "dead zone" where the aliens would be unable to deploy without dying immediately from the high radiation, or at least losing all their electronics. $\endgroup$
    – DevilApple227
    Commented Sep 1, 2016 at 16:32
  • $\begingroup$ Whoa I wrote this so long ago! - Nuking them right away would not be possible as we will not now their intentions right away. If we let them spread a lot, then those nukes would be a bit ill-used. - When you detonate a nuke on the surface(what we will need to be sure the portals are down) you create no EMP. EMP comes from detonating a nuclear device at an altitude. - Every being has a radiation poisoning threshold, we will be unable to tell if rads will have any effect on them whatsoever until after we try it(immediate vicinity enemies would be dead tough as nothing can survive). $\endgroup$
    – Konstantin Velickovic
    Commented Sep 2, 2016 at 22:15
  • $\begingroup$ I think that defending against them would be pretty impossible without a first-strike. Also, what kind of intentions can you imagine from a highly armed contingent of forces appearing out of nowhere, bearing no warning. If it turns out we annihilated a peaceful race, well, sorry, but better them than us! And if you throw enough nukes at something, you're certain to get enough radiation to fry pretty much anything. $\endgroup$
    – DevilApple227
    Commented Sep 3, 2016 at 13:19
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A few considerations for those of you have answered so far: as far as infantry is concerned, you have to realize that one of the chief concerns for modern infantry is twofold-

First of all, let's make some distinctions in what, exactly, infantry means: Some things to note, is that historically, there have been many different "classes" of infantry

  • Light Infantry - Historically a fast, rapidly deploy-able force with much lighter armament (i.e. a light infantry platoon would only have one weapon of the same classification at the .50 BMG whereas a Mechanized platoon could have much more)
  • Heavy Infantry - slower to move but usually backed up with better supported fighting positions and supply lines. These ranks could conceivably have much heavier weapons with sufficient amounts of ammunition being supplied by support elements (trucks filled with ammo)
  • Mechanized Infantry - Armored troop carriers and heavier equipment tables.

Even under current military equipment tables, the armament and equipment each class of infantry brings into battle depending on the respective class and roles in modern combat.

So take into consideration that to have every single soldier who is on foot (light infantry) armed with a .50 BMG is, quite frankly, not likely. Since the Browning is what would be considered a "crew served weapon" platform (too heavy for one man to carry all the stuff necessary to make it an effective weapons platform). Unless they were using some form of powered exoskeleton would more than likely have to forgoe a ammo hogging weapon system like the BMG and use a comparable caliber rifle with sufficient penetration like a elephant hunting rifle with insane rounds like .577 Nitro Express or .404 + Magnum rounds.

On the flip-side of this, heavy infantry could easily set up entrenched fighting positions (machine gun nests) with BMG's as long as they have a steady supply stream coming in and they stay in the same position.

Mechanized Infantry will be able to carry heavier equipment but still requires time to set up the weapon platforms after exiting their armored transports. They will also have the luxury of having heavily armed and armored vehicles supporting their movement.

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Every warrior has a weakness, a chink in it's armour. These subterranean creatures come from an underground environment with a constant warm temperature, I doubt they would have evolved (if they reproduce underground) to withstand temperature extremes. Here are a couple of options to exploit this weakness;

Liquid Nitrogen

Most materials when submerged in, or exposed to liquid nitrogen will become extremely brittle and shatter on subsequent impact. This weakness could be discovered by a protagonist using a CO2 fire extinguisher for defence in a close quarters encounter, or by observing limited abilities of the creatures at higher latitudes/altitudes where the temperature is lower. Ultimately the military could develop "freeze throwers", LN warheads or modify the water bomber aircraft that put out forest fires to drop liquid nitrogen (if enough can be produced) I assume the aliens can't fly

Directed energy

Based on the same principles, microwaves or other radiation beams could be employed to cook the aliens from the inside out. The white (and yolk of an egg will solidify at reasonably low temperatures if maintained for long enough. Perhaps the alien's blood can be coagulated easily by heating it. This is the opposite extreme temperature vulnerability and could likewise be discovered by our hero cooking an alien with a HTHW or steam pipe, or by observing limited abilities around the equator. This could also be backed up by the aliens mysteriously avoiding certain continents depending on the time of year and seasons. Ultimately the military could have, or launch orbital satellites that cook the aliens like ants under a magnifying glass using directed solar radiation. I assume the aliens have no presence in space/orbit.

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The main issue here is that the human soldiers don't outnumber the aliens by that large of a margin. Also note that M60 system typically requires at least two (human) soldiers to operate (one to fire and one to carry ammunition). You've said that an alien is physically stronger than a human, so let's assume that gives an alien with the M60 and its advanced armor the same mobility as a human soldier. These facts together don't bode well for humanity.

One option would be to simply mass produce .50 caliber anti-materiel rifles and ammunition. Presumably the aliens would modify their tactics and/or loadout if humanity went down this path.

Grenades, mortars, RPGs and other portable explosives would be a decent counter to the aliens, depending on the properties of their armor (as has been pointed out, flamethrowers have limited range and may be a hazard to their operators and fellow soldiers if the fuel tanks are compromised). Carrying ammunition would be the primary issue. Potentially, rapid development of the Boston Dynamics LS3 autonomous pack robot would mitigate this.

More generally, introducing a large number of decently armored drones/robots into the combat environment would serve as shields/decoys for human soldiers and force the aliens to use their anti-armor missile systems more. In turn, this might give tanks and artillery more survivability in the field. Ultimately, the action on the ground will mostly serve to delay, rather than defeat, the aliens. As others have pointed out, the human air force will likely be responsible for the most alien deaths, assuming that the human infantry can slow the aliens' advance.

Chemical and biological agents could be considered, depending on how well-sealed the alien armor is. Neutron radiation could also be considered, but if the alien armor contains shielding (like borated materials), it may not be effective. And in any case, these options may not be feasible for infantry to implement.

All this said, if the portals the aliens use for resupply aren't destroyed, humanity faces a war of attrition that it probably will lose (unless the total number of aliens is some fixed number much much less than the population of the Earth).

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RODS OF GOD

Since your aliens are stuck on the ground, and nukes might have dire side-effects, consider rods of god from space. Also known as kinetic energy weapons. While most depictions are of satellites with large armaments of tungsten rods, consider using high altitude drones carrying these rods--perhaps a solar-powered army of drones with magnetic rail or coil guns for the rods. This would be an effective way to neutralize these enemy forces on the ground.

http://transmissionsmedia.com/

According to Popular Science:

If so-called "Rods from God"--an informal nickname of untraceable origin--ever do materialize, it won't be for at least 15 years. Launching heavy tungsten rods into space will require substantially cheaper rocket technology than we have today. But there are numerous other obstacles to making such a system work. Pike, of GlobalSecurity.org, argues that the rods' speed would be so high that they would vaporize on impact, before the rods could penetrate the surface. Furthermore, the "absentee ratio"--the fact that orbiting satellites circle the Earth every 100 minutes and so at any given time might be far from the desired target--would be prohibitive. A better solution, Pike argues, is to pursue the original concept: Place the rods atop intercontinental ballistic missiles, which would slow down enough during the downward part of their trajectory to avoid vaporizing on impact. ICBMs would also be less expensive and, since they're stationed on Earth, would take less time to reach their targets. "The space-basing people seem to understand the downside of space weapons," Pike says--among them, high costs and the difficulty of maintaining weapon platforms in orbit. "But I'll still bet you there's a lot of classified work on this going on right now."

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    $\begingroup$ Deadly, yes. Able to be quickly deployed in sufficient numbers, no. $\endgroup$
    – Loren Pechtel
    Commented Nov 27, 2014 at 2:07
  • $\begingroup$ Rods from God aren't really very effective; it takes a huge amount of energy to launch them into space, and you also need an equally big push to make them come down again. $\endgroup$
    – In Hoc Signo
    Commented Mar 26, 2020 at 19:55
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You are describing aliens too tough to kill by most anti-infantry weapons, but they aren't in large numbers. Thus you're describing battlefield armor--go after them as you would any other battlefield armor.

1) Anti-tank missiles. You didn't list them as having smoke systems, they'll have almost no chance against something like a Hellfire. The helicopters that fire them will be beyond the range of the alien's weapons and they can be guided to their target by a helicopter hiding behind terrain and showing nothing but the laser pod that sticks up above the rotor. The helicopter that fires need not be the one that lazes, either. A well-hidden soldier can also operate the laser.

Modern anti-tank missiles can defeat most any armor that you can put on a tank (the defense against them is to either hide from them or knock them out of the sky just before impact with reactive armor.)

2) I doubt the infantry anti-tank missiles will track but anti-tank rockets will work, although they will be dangerous for the people firing them as they don't fly too far.

3) Take a page out of Desert Storm: Tank plinking. A fighter bomber goes up with a load of laser-guided bombs, find a target, laze it, drop a bomb.

Since you are describing a force of pure infantry they'll be shredded. At least in Desert Storm the Iraqis had some anti-air, your aliens don't.

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Lots of people have already discussed this question, but I want to look at it from a slightly different angle. Why would aliens choose to deploy on the continents? The problem with humans is that we have exactly one target worth defending: Earth. For our purposes, we have no idea about where their home planet is, or how many colonies they have. Our current weaponry is only good for a one or two dimensional attack, the one we face on Earth. On earth, latitude and longitude are the only viable ways of maneuvering. You can't really burrow too deep in the earth, and even if you use planes, in the end, you end up approaching on a 2 dimensional plane. On the other hand, earth is a large, 3 dimensional target. They can just decide to ignore our space defense fortifications, by flying around on a different plane. Earth just doesn't have enough resources or military might to defend from all directions. For that reason, they don't need a ground invasion. They can just approach from the undefended directions, and hold the entire earth "hostage". They send a message to all the major powers, saying that unless they immediately dismantle the defensive facilities in space, they will launch a bombing attack at all the major cities in the world. Earth will have no time to react, as it only takes a few minutes to drop missiles, and a matter of seconds to fire a laser or similar beam weapon. Not nearly enough time to deploy a anti-bomber force of fighters and such. Similarly, if Earth decides to fire ground weapons, such as ICBMs and other very long range weaponry, the aliens will still have several minutes of advance warning, more than enough time to bomb Earth into oblivion, perhaps even enough time to warp back out of range of the missiles. That's not even mentioning the fact that they may have counter-missile systems, and simply shoot down the missiles, or maybe they can jam their navigation systems enough that they can avoid them.

In short, there isn't much Earth can do. Earth can either increase their defense of space (which in the current day is non existent, but I'll assume some exists in the scenario) 1000 fold, or they can practice a policy of first-strike and early detection, meaning finding the aliens first, having ships that warp to their home planet, to set up a similar hostage situation, or launching ICBMs the second that any ship signatures are detected, without pausing to negotiate. Even in that case, its possible, that the aliens would still be able to destroy earth. In that case, you would need to set up stations with rail-guns in such a way that any point in space is within a mile or so of a rail-gun.

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  • $\begingroup$ Detecting a ship doesn't matter if that ship is a string of asteroids. You don't need to be seen to be able to broadcast a radio signal and once you've nudged those 20 massive asteroids just that tiny bit that will have them crash into Earth in 20 years time, humans can't do squat to stop it. $\endgroup$
    – Clearer
    Commented Jan 23, 2018 at 7:35
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First Step: Lose

There's little chance for the humans to win if the aliens are smart enough to master teleportation and have access points in all continents. They will quickly realize the pressure points in the human military machine and will use their superior maneuverability to cut out any supply train.

All the mighty M-1 tanks? Still in their garages waiting for fuel that will never come.

All the Nukes? Still in their silos, waiting for a target that never stops long enough in a place to be targeted.

All the soldiers? Giving away their guns to the aliens in exchange for food as the winter gets worse.

Second Step: Adapt

So, now Earth is under Alien control, its time to learn about them. With some luck you will have hidden bunkers of surviving scientists and specialists that can be supplied by the rebel forces in the occupied cities.

The main question is: Who are our new conquerors?

We will need to learn all we can about them in a short time span, specially what are their plans for humanity and if there are other races that have been put under their control before.

The old world order has failed, so its time to learn with our conquerors how they came to achieve it, what they have done better than us and how we can use that for our own means.

If humanity is to survive it will need new technology for a new kind of war, it will need supplies, and above all it will need allies.

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Quick answer, chemical weapons, explosives, and nukes. They may assume that we don't have chems because they are never used (other then the police force using tear gas), explosives are just hard to resist, and nukes are, as far as we know, impossible to resist, if the explosion doesn't kill them (which is near impossible) the radiation surely will.

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You describe aliens that are armoured to a degree that infantry rifle ammunition will not penetrate, but can be penetrated by an anti-material rifle, probably .50 calibre, as that is a typical round used by such weapons.

.50 cal and larger anti-material weapons may be issued to infantry soldiers in greater numbers, but their numbers would be initially limited, and a large anti-material rifle has the disadvantage that it cannot readily be fired from a standing position.

To cover the need for a shorter-ranged weapon that would be effective against these aliens, and is available now in significant quantities, let me suggest the 40mm grenade system. 40mm grenades are in use by a wide variety of military forces around the world, 40mm grenade launchers are available that can be attached to existing infantry rifles or as stand-alone infantry weapons, and are also available as larger, crew-served belt-fed weapons.

The 40mm grenade system includes a number of different rounds for different purposes, but includes the M433 round, which is a HEDP explosive round that can penetrate 50mm of hard steel armour plate, as well as causing casualties from shrapnel. This can be fired from any 40mm grenade launcher.

M430 rounds are higher velocity rounds that are more limited in the weapons that can fire it, and can also penetrate 50mm of hard steel armour, or in their M430A1 variant, 76mm of armour.

Given the high availability of these weapons and their ammunition, and the fact that ammunition exists that should be capable of dealing with the armoured aliens in question, would make them an ideal fall-back weapon that can be issued quickly to deal with the threat.

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