How does a mundane army defeat an immortal army?

Question

In trying to both invent and prevent mediums with which opposing armies can defeat "Army X", I come to you, stack exchange, hoping for some second-opinions on how to 'defeat an immortal army'.

In essence; "How does an army equipped with standard blades and shields (and the etc that comes with it) ultimately defeat, or otherwise fend off invaders who are immortal?"

I wish to find novel ways to deal with them, or to patch up loopholes, that I'd like to see removed, with new conditions.

Conditions:
1. Army X is immortal. Its soldiers can be 'killed' temporarily, but will be spontaneously revived after a few seconds where they fell with their wounds removed, including but not limited to dismemberment, beheading, etc.
2. Those individuals in Army X who find their bodies 'stuck' (such as in a pit, chained, etc) can instead opt to resurrect inside of a suit of armor elsewhere in the world. The same goes for bodies otherwise completely and irreversibly destroyed. Note that said armor is specially designed and constructed, meant solely to house said dead warrior of Army X should they be faced with a trapped original body. Said armors are located only in their capital city. A warrior of Army X that has undergone this transfer in no longer flesh and blood, but rather a walking animated suit of armor from then on.
3. The medium with which Army X maintains their immortality is through a single individual living within an 'impenetrable city' (which could be a discussion for another time). For all tends and purposes in this question, this healer cannot be killed, or their powers otherwise nullified.
4. Each member of Army X is, in essence, a 'hero'. They are the sort of character capable of fending off a great number of enemy grunts single-handedly. No other nation possesses the capacity to create anywhere near as many soldiers of the same caliber.
5. Though the enemies of Army X do possess the capacity for spell-slinging, for this question consider them incapable of magic; I'm looking for other solutions to defeating/fending off the army than magic.
6. Soldiers of Army X do need food and water, but can be revived regardless of death from starvation/thirst.
7. Soldiers of Army X die from old age, and are not revived upon doing so. They cannot, however, be afflicted to grow older faster (due to magic, for example).

To reiterate
What sort of tactics/strategy could an army with mundane medieval weaponry utilize to defeat or otherwise fend off invading Army X?

EDIT Based on comments/responses, I'd like to point out that the apparent unstoppability of Army X is completely intentional. Army X is meant (at least, for a time) to be unstoppable. For the world in question, their unstoppability does not last, but I wanted to explore any possible ways I hadn't considered before that they could still be defeated. Condition 3, namely, is the medium by which their unstoppability is lost.

Answer 1

Hope you invested in the sciences before you were attacked.

The trick to defeating this army is to incapacitate without inflicting lethal wounds. You could maim them, but that would still give them the opportunity to suicide and spawn again. So, how do we defeat a hostile force without inflicting bodily harm?

Incapacitate them. Something that is fast-acting and can be applied on the battlefield, that persists after initial exposure, but doesn't cause undo harm to our own troops or generally have a lethal effect on victims.

Chemical weapons come to mind. But not something intended to be lethal, like mustard gas or the other ugly compounds banned by the Geneva accords. What about a sleep agent? Chloroform has gained notoriety in crime dramas for quickly putting people to sleep, but the science doesn't support that.

But chloroform as we have been led to understand it acts like a particular other substance, namely, sleeping gas. Sleeping gas is a generic term for an inhaled substance that induces an unconscious state in victims, and there are plenty of real-world examples for this. BZ gas, for example, is a military-grade sleep agent. Of course, being military grade, the production process isn't public knowledge.

Instead, let's look at methoxylflurane. This gas can induce pain relief in 3-mL doses for up 30 minutes at a time, and sedation in 6-mL doses. It takes just 6 to 8 breaths for effects to kick in. So, if we were to construct a projectile weapon containing methoxylflurane and unleashed it on the army, we could incapacitate huge swaths of the invaders.

Once the soldiers are incapacitated, we can tie them up and ship them off to parts unknown. An army of 5,000 isn't an army if each soldier is in a different location.

Mandatory Caution Notice: Fluorine is a dangerous substance that likes to react with just about everything. Many researchers died trying to isolate fluorine due to complications with hydrofluoric acid. Researchers and soldiers alike should take precautions when dealing with methoxylflurane.

Mandatory Warning: Some subjects of methoxylflurane may die due to complications arising from overdosing and medical preconditions. While collecting the incapacitated, be wary of soldiers feigning sleep or rousing early.

Answer 2

Given everything that you've stated... You don't.

With them reviving a few seconds after death, this means that any sort of "close" battle - The only real option - is a defeat for normal troopers.

Consider this: Normally, you could sit inside your walled city and be... OK-ish. With Army X? They can literally just take catapults and fling their guys over. Sure, they'll die. Then they'll resurrect.

You can defeat individuals via capture, but you simply can't capture an entire army that doesn't want to be captured. A single immortal who cares nothing for death could kill many, many normal people before they get overwhelmed and captured, and that's a war of attrition the normal people cannot win. The immortals are at full strength pretty much all the time, while normal soldiers have to be able to resupply their losses.

Now, if the Immortals revived after hours and/or something a lot easier like decapitation forced them to revive at their home city, you could push them back by forcing them to revive significantly further away. You'd also be able to steal their stuff, which I'm assuming isn't as easily replaced.

Answer 3

The only viable way is to destroy or circumvent the command and control system.

Your soldiers are (ridiculously) hard to destroy, so they're simply not a viable target. Targeting them would be to attack the enemy at the strongest point, which is bad tactics.

There is, by the way, a serious flaw with them : if you can't kill them, it will surely become apparent to the soldiers themselves that they should be giving orders, not taking them. What happens when they run out of "official" enemies ? These types aren't going to all retire to gardening and church socials, they're going to want to take over. You'd quite likely end up with a lot of warlords fighting endlessly with each other and rife with internal dissent.

But they have to be controlled, or they're just useless grunts. Someone has to direct them in a strategic way : what do they do and who to.

So you'd use covert or down-right sneaky methods to infiltrate and attack their communications. You'd ideally attack select senior commanders or the political leaders. You'd interfere with their lines of communications.

And even if they are willing to die because they instantly get recreated whole, there's a difference between dying repeatedly by desperate enemies who have no option but to fight you, and being willing to suffer all that pain and misery for an endless battle. This is particularly the case when they split into factions and start internal battles.

And note that because they are essentially invincible, they'll rapidly start to view everyone else as irrelevant, slaves at best, useless at worst. That's another problem : they're a warrior caste and the only worthy enemy for them is their own kind. Another possibility is that, as a warrior caste they actually develop an ethic that's it's dishonorable (or similar) to battle mortal opponents. That might limit their willingness to fight.

Likewise, killing and being killed is a messy and emotionally exhausting business, even if you're invincible. Killing mortals is pointless - it's just easy - so will they suffer battle fatigue ? Even the best soldiers can suffer battle fatigue. Will they start asking questions like "why are we doing this ?". Maybe they will want to retire to more productive lives. In WW1 there was a notable instance where the exhausted (but still brave) French army went on strike for better conditions (which they got at some sacrificial cost). Would a peace movement break out ? There were many instances of front lines developing a live-and-let-live mentality : we won't bother you if you don't bother us. During the American Civil War there was a case where soldiers declined an opportunity to shoot at opposing officers in the distance (out inspecting their own lines) because they regarded it as murder. Would your soldiers start viewing killing mortals as plain old nasty murder and simply stop doing it ?

Again, maybe you can "turn" them (or enough of them) with some cleverly directed propaganda. Maybe you'll convince some units will decide that defending mortals is more honorable than killing them. Maybe you'll convince them that you're no threat to them (and you're not) and they'll look for more challenging enemies (other immortals). Maybe you'll simply bribe them : we'll pay you more than the idiots you're working for now, wine, women, song.

So there are ways to attack them, it's just that the weapons you need are not blades, but perhaps words.

Answer 4

Three ideas spring to mind.

• One - Psychological warfare.

You win if you convince them to stop attacking you. I have no idea what this would look like in your world, but mind-games, propaganda, illusion magic, diplomacy, or lies - any of this could be an effective attack vector. Find their fears and prey on them, or find their desires and tempt them. Give 'em what they want and hope they leave. They do have a goal, right? As long as it's not 'eradication of all life', giving it to them is obviously worth serious consideration.

• Two - keep them alive.

So, they can respawn elsewhere if their dead body is trapped. Alright, but can they kill themselves? If you can capture them while denying them the ability to respawn, they're stuck until they die of old age. Drugs and maiming are your friends here. After their capture, I'd suggest combining this strategy with point one above, and attacking their mind while you have control of their body; addiction, brainwashing, sensory deprivation, or just good old torture... It's definitely possible to break some people's will, and although their body might be immortal, it doesn't do them much good if they spend the rest of their life cowering under their bed.

• Three - build a better wall.

So each of them is a hero - okay. But the defender has a definite advantage, and unless they have unlimited resources and strength, there must be barriers they can't overcome. Use a mountaintop castle or a lava moat or a flying island. Move off-planet, move to a different dimension. They respawn elsewhere if their entire body is destroyed, so the most you have to do to repel them is create a defense that guarantees the destruction of their entire body, make sure you have no viable respawn points inside, (store all armor in separate pieces) and they won't be getting in.

Well...

There are possibly more solutions, depending on how you define 'wounds', 'armor', and 'total destruction of their bodies'. (Ship of Theseus arguments.) Is being sliced in half a wound, or total destruction? If they respawn without wounds or scars, do they respawn without immunity? Is being sick a 'wound' they can heal from? Using plague that you're immune to but they're not might be worthwhile, even if it just weakens them enough to make capturing them easier, for points 1 & 2. Would bleeding to death from ebola count as 'total destruction'? Infect their water. Or maybe a viciously painful poison on your weapons would work, something like box jellyfish venom. Even if they kill themselves, as long as the poison remains, they might not be willing to resurrect in the same body.

Answer 5

The first step here is to remember the single most important rule of war. No other rule is more important:

The goal of war is not to make your opponent lose. The goal of war is to make yourself win, and barring that, make yourself not lose.

Your mundane army surely has a purpose. They have something they want to fight for. They probably want to get back to farming their fields and raising families. If your mundane army's goal is to make Army X lose, then they have failed the first rule of war, and are going to lose no matter how creative we get.

Got it? Good. Now, there is a loophole to work with here. Their respawn rules are simple:

1. They can respawn in the same body
2. They can respawn back in their city
3. Where they respawn is their choice

This provides the natural solution: we need to make sure that infinitely respawning warriors at those two locations isn't unacceptable. Make sure you can achieve your win condition despite an infinite army respawning and then go about your business.

The simplest solution is to pin them back into their city. This wont be easy. You might try grappling them rather than killing them, so you can push them backwards or drag them back to their city. Breaking large numbers of bones may be an effective approach. Whatever approach you take, you want to reduce the area they have to work with. This limits their resources. Once you have the resource of the entire shared kingdoms of your planet, and they have the resources of a single city, you are no longer on such shaky ground. Now it is time for a new rule. We started with rule number one. Now we'll skip a few, to rule 37:

Rule 37: There is no "overkill." There is only "open fire" and "I need to reload." - Schlock Mercenary

The city may be impenetrable, but the surrounding land isn't... yet. Your job is to weaponize a kill zone so ungodly that no Army X soldier will ever get the privilege of respawning within the kill zone. They will be obliged to respawn back at home base. I recommend lots of flamethrowers. Maybe some really big mosquitos too, if the flamethrowers aren't cutting it. I hear Minnesota has some good ones.

From there, war just becomes part of life. You have a general rotation of soldiers which man the flaming-killzone-of-doom for a few months and then rotate back. You have an entire world to police with.

From there, it's up to you as the builder of this world. How much can life suck for Army X before they decide maybe they should talk to us instead of fight us.

Answer 6

I am drawn to this key word and tricky phrase:

can instead opt to resurrect inside of a suit of armor elsewhere in the world.

---

‘Army X does indeed have one weakness, m'lord: Those suits of armor which they wear seem to be the source of their vital powers. Removing the armor from one living seems impossible by our assessments; if, however, we can distract the army long enough to sneak a few of our stealthiest soldiers behind their lines, we might be able to discover a way to destroy their foundries and reserves of armor.
‘Then, it is simply a matter of trapping the others in lidded tanks filled with oil.

‘Our losses will be numerous, but the sacrifices must be made. They are, after all, peons, and we are nobles.’

Answer 7

I'm not exactly sure how aging affects your soldiers, but I always thought humans don't die of age but of your bodys failure to cope with all the stuff that's killing you. Diseases, Injuries, damaged cells that can't be copied any more, etc. I'm not a biologist so take it with a grain of salt. :)

Since they can revive and regain lost body parts I have to assume it is a rather crude will based process and not based on "last state of your body before death". If it was they would not regain body parts lost before their death, but only those causing it. If it was not "crude" they would be able to fix the damage aging does too.

If it was some kind of save point they create of their body before death to regain once they died you got a leverage here. They'd loose memory of the time after creating the save point once they respawn. Since their brain would regain it's former state. Crucial information gathered that must not be lost could force them to "save" even when wounded. But I'm not sure how that process works for you so I won't detail this further.

Can they get sick? If they were struck with an infected blade, would their revival still have them carry that infection? If they can, you could infect them with a disease that weakens their bodies. Not killing but weakening and reducing morale. If it is a bacterial infection you could argue that it does not get removed as it is a live form within them and no wound.

Since they need food you could also poison that. Nothing lethal but more crippling things. Maybe something that makes it hard to breathe, resulting in severely weaker troops. They'd have to suicide to get healthy again, if revival even removed the poisoning.

Knocking them out. You could just use something non lethal to knock them out. Gas maybe? Then feed them with means that allow them to stay unconcious. IV bag maybe? :) Then just feed them for their lifespan in a big prison like the matrix. The matrix (or something similar) would be viable too, but since you wanted them to fight with swords, shields and magic I assumed the setting would be more medieval. It doesn't stop you from using hallocinogenic stuff though. Make them think they fight/whatever by using alchemy/magic/drugs. Since they eat and drink you could use drugs as an alternative for poisoning the water/food.

Does respawning clear addictions? Be cruel and make them addicted to something only you can provide. A formula only stored in the head of an alchemist? They can't take it by force (if the alchemist does not succumb to torture).

Answer 8

Yeah... A mundane medieval army is at huge disadvantage against this Army X. It's just not possible to do any lasting harm to them or deprive them of any significant amount of important resources. And there's no magical way to stop them, like summoning a bigger fish or holding the whole world hostage.

Which leaves few paths to the victory open, one of them being to

Surrender to the Army X

And after that, use non-military means to make holding you and your resources bad for them.

If those sword and shields can't bring you a victory, just don't use them at all, and don't waste your people in the process. Use them in another way, applying social and economical pressure against the Army X that the winner of the war wouldn't be able to avoid.

Like:

• Assimilate Army X "heroes" into your culture. So in the next war, they'll actually be fighting on your side.
• Steal their young. If they have to live among you to control the conquered territory, take their children and raise them to fight their parents.
• Organize ongoing non-violent civil resistance movement against Army X. Let your people refuse to obey any laws of their making, any orders that they give. Make them spend lots of time and effort to get anything done. Manipulate them into committing atrocities against your people when they try to enforce any semblance of order. If they are civilized and enlightened, they won't be able to handle it, and they'll go away by themselves. If not, the mages of your world may come to your aid, so the Army X advantage against you will be neutralized.
• Sabotage producing any food on your land. Make Army X feed your people, or face the unpleasant consequences of the previous method.

Answer 9

Assuming that the mundane army is forced into conflict… I'm sure those tactics could be more or less handled, still I have to state how one would try to deal with immortal warriors. While in given situation such an army would wipe out any enemy in open battle, the weakness of such army still lies in a few factors, which can be used:

• Stamina, sooner or later they will need to rest. That is when they are most susceptible for attack. Their ability to act will be also reduced when exposed to starvation or/and dehydration. The point is to capture them before they will die.
• Immortality, make them wish to die, cripple them in any monstrous way it's possible to imagine and allow them to get revived in their capital as useless mindbroken wreckage if magic will not counter any mind-shattering. Such soldiers will be useless and will not be able to reproduce as they will be just set of fancy armor, meaning less heroes to be born. The matter of question at this point is: do those 'living armors' feel pain or fatigue? If not, it would be easier need to keep captive soldiers alive just without limbs to make sure they will not escape as 'living armors' that would still be able to act at war would definitely seek for revenge, while flesh'n'blood soldiers would be more likely to distance themselves from danger of sharing fate of such an armor.
• Subjugation, including mind-control, brain washing, psychological warfare and similar techniques. Even in medieval times there were effective techniques of breaking one's mind and fighting spirit. That technique would prove especially effective against 'living armors' that would be captured in ambushes. If armors wouldn't 'feel' it could be easy to cut off their vision and hearing by closing their heads (meaning helmets) restrained in some containers. It could prove useful due to fact that space needed to constrain such an armor would be comparatively little (helmet needed) and would quickly lead living armor's mind into madness as people are unable to psychologically withstand sensory lockout.
• Slowing down, while immortal immortal army still has shape close to human so it's possible to catch one in standard pits and traps. Also even living armor will have to slow down when traveling thorough difficult terrain.
• Economy, production of armor and equipment that may be ditched at will, especially when it's magical is costly. Whether one would be able to subsequently catch enough of immortal warriors it could disrupt state's economy while providing (at worst case) recycle material for it's enemies.
• Destruction, use swift attacks to destroy lands and villages of enemy to destabilize lives of their citizens. Calls for retaliation unless destruction is more localized and performed by spies. Agents may destroy immortal army support and disrupt it's standing in state or command chains.
• Infiltration, get access to vulnerable points and attempt to destroy enemy from inside.
• Politics, maybe no other nation may have so many heroes, but how about all neighbors?
• Research, capturing enough magical armors create own or hijack captured ones into battle golems in attempt to effectively out-tank enemy or create own immortal warriors only with more 'respawn' mages.
• Burn it all, when mentioning magical armor, it means there is some magical prowess. Use battle mages or other means of effective and complete destruction. While warriors may be immortal, they may be forced into respawning as armor so they will need some time and means of transportation to come back at battlefront.

Note, that proposed ways of fighting need ways to perform backstage actions, stability or possibility to equalize chances of fighting by reducing numbers or strength of enemy dealt at once.

Edit: In case it wasn't clear, due to ability to jump into 'living armor' when shackled (let's assume transfer may be performed at will) it becomes nearly impossible to capture and tricky to torture immortal soldiers unless transfer may be somehow delayed. Still it could be possible to disable armor senses by taking off it's helmet and this way force transfer, which could be disabled by ability of sorts 'phantom body', which would need only a little tweak to the general idea of living armor.

As for killing the most important pillar of immortal army, it is sure that all the countries would attempt to kill one on the spot whenever the information would leak to them. Still nobody said that there is really only one 'respawn' mage and as sad as it sounds it calls for real-time soul-crashing defeat when mundane army gets to feel the victory in their grasp… As nobody is really irreplaceable.

Also as it was stated by M i ech there are many ways to make advantage of immortal army setting to make it even worse enemy unless one will successfully make them turn against their master who would probably would just need to revoke their personal immortality and immortalize some other citizens. This makes coup d' etat a bit more tricky than normal.

Answer 10

As other answers put it, mere mortals can't win (in this battle). So don't give battle. War is politics with weapons, according to Carl von Clausewitz. So keep politicking without weapons, as long as needed. Intrigues anyone?

But you didn't tell why they fight. Nor the political organisations. So this is all I can tell.

Answer 11

Radiation induced Cancer

Since the only way to get your soldiers to die is of old age, then you have to kill them with cancer or heart disease. Force-feeding them bacon doesn't seem like it would work that well, but cancer is the kind of thing you can non-consensually inflict on a nearby immortal soldier.

If you have wizards on your side (which I don't see why not, immortal armies seem pretty magical) then you can heavily irradiate the enemy soldiers. Send your soldiers into battle with blade tips magically coated with something radioactive and nasty (polonium?). A few stab wounds, and some deposited radioactive materials, and then beat a hasty retreat. You can expect the immortal soldiers to die of 'old age' in a few weeks. Tritium would do the trick, or Caesium-137, Iodine-131, and Strontium-90 all have nasty cancer causing effects.

Alternately, you can use a spell of 'Summon Proton Beam' to irradiate your enemies from afar, preferably the safe confines of your castle walls. The same kind of beams that are used for radiation therapy can be pretty effective if directed at important things you only have one of, like your liver, or brain. If wizards are expected to throw fireballs and summon lightning, I wouldn't consider gamma ray sniping to be too outlandish.

Answer 12

Hurt them.

So they can't be killed, or they respawn. So they will heal of injuries when they respawn. So they can't be trapped, because they can escape while respawning.

Do not let them respawn. Keep them alive. You do not need be so merciful.

Cause, well, there's nothing at all, at all, in the rules that say it doesn't hurt to be injured, to be trapped, to die. Nothing that says injuries to the heart or mind or soul heal like injuries to the body.

Practically, you don't fight them head on, you ambush scattered groups in any way possible, at overwhelming odds and with any trap available, take prisoners, and move on to capture the next lot. And you take them prisoner, trap them living, strap them down, and hurt them. Can you induce PTSD? Can your traumatize them? Break their morale, their courage, their confidence?

Can you do your level best that, if and when they slip out of your hands and respawn far away, they will look at their leaders exhorting them back into battle with you and despair? Or maybeso hate those leaders driving them back to that duty, that risk? Or fear falling back into your hands?

You can't permanently take them off the field, but you can do much to make them want to take themselves off. Make them reluctant to go back to war, hesitant (or at best, incapable due to trauma) of going back to the battlefield. They revive whole, so they wouldn't "need" that much of a reprieve before being sent back within reach of their torturers.

To be fair, this is going to also make them hate you, and want to hurt you back. They will have no mercy, not after this. But if it's the only chance you've got, if you know how to hate and how to fear, maybe you can persuade them you're not a palatable target, you can make yourselves more trouble than you're worth. Maybe.

As a bonus, give them a way out... maybe if they surrender instead of fight, they get time at a decent POW camp, instead of the torturers. Maybe if you find deserters of theirs, you treat them decently or offer shelter. Let their foot soldiers wonder, at every ambush, if they should lay down their arms and be treated well, or stand their ground and, if loosing, suffer for it.

And maybe do something extra about higher-ups... maybe lighter treatment, or automatically sent to the decent POW camps, so their foot soldiers resent those ordering them about, safe and risking little while their own suffer. Maybe, if you're lucky, convince the officers that you don't treat them all that bad, maybe those foot soldiers who reported otherwise were malingering. Or maybe very carefully separating them from their soldiers, never talking about what happens to them or admitting that you have them at all, and gently and with all healing skill crippling them or sending them into comas, so their officers once taken just disappear and are not heard from again. Teach them fear, when they don't know what's happening, when they don't know why their officers never return or if you found some way around the magic respawning.

Answer 13

As multiple other people said, you don't.

Worse. Depending on details of respawn mechanism, you can never defeat them. Not even with your plot idea.

You imply, later on you will assasinate the being that heals them. You won't.

You said they respawn in seconds. If they find themselves trapped, they can ditch the body and respawn inside armour. Problem is, if they decide what means trapped, they can pretty much opt to just respawn themselves instead of walking down the hall. Because it's faster that way, system like that begs to be abused.

In this part I'm assuming you mean specially designed and built magic armour.

In that case, they have ultimate strategic mobility. It's as easy as building armour to easily open in front, so respawnee can walk out and they can easily hop to any place they can bring armour to. One army can handle all the fronts at the same time - while enemy will spend weeks marching, they can just station army in capital, and ferry wagons with respawn armour everywhere. As soon as they near enemy, army hops to the location, and after they drown enemy in their bodies (don't care about attrition), they hop back. They don't even need weapons. They just need to rush enemy, dying over and over again, swamp enemy and take their weapons. Doesn't matter if they die thousands times each before they steal one sword, they are still making progress.

This is also why you will never defeat them. All their locations are defended at all times (stock weapons in cities to avoid wasting time on stealing enemy weapons) and they can rely messages instantly. You will never get close to the capital. You have no chance to sneak small group to attack the capital, because you CAN'T tie their forces at all and if you somehow coordinate attack, hundred kilometres apart to try, they just need to send one respawnee to recall forces back to capital - they don't care about losing ground, they can take it back without effort. You can't assassinate that being, because there's no reason to ever have non-"immortals" in same district, or even same city as that being - Immortals can be servants while they are not actively fighting. Use spwan-hopping to handle diplomacy, if you ever want to talk to the inferior kingdoms, and make it crystal clear that envoys stay the hell out of capital.

While in this part, I'm assuming you mean any armour.

This is something that turned up in comment discussion with Cort Ammon. If you mean they can spawn inside any empty suit of armour, or channel Agent Smith and take over body of anyone wearing armour, then it's even worse. Your Army X wins war against everyone before anyone even knows there's a war. As soon as being creates first regiment, they perform decapitation attack on nearby kingdom. Since they have FTL communication (using respawning couriers), while everyone else has to rely on couriers on horses, they can simply decapitate next kingdom before news even reach them. Thus, they can decapitate every kingdom in the world (all nations used some form of armour, at least ceremonial and as status symbol, good enough for Army X) before anyone can even figure out what's going on. If in your world there's magic allowing to talk at distance, other kingdoms may get a warning fo Army X, but they still won't have time to muster any form of defence - calling together feudal armies tended to take a lot of time and any suit of armour, in every feudal lords armoury is a spawn point - if King gets warning and somehow manages to get his subjects to disassemble all armour in his castle, all his vassals can still be subverted and used to stage assault on capital, when convenient. No hurry. They have probably about a month to decapitate as many kingdoms as possible, before they have to worry about those who escaped initial strikes.

Reading your question, I think it's even worse than you planned for.

Answer 14

So your Army X cannot be defeated in combat.

Let's face it: it just won't work.

If cou cannot win a fight, don't fight it. It's as simple as that.

You could offer them something in return for not having to fight.

But okay. You want a battle. Because we all know that you cannot have a story about an invincible army if you don't have a fight. Narrativium demands it, so we have no choice.
Well, of course we do have a choice, if only a choice of weapons.

Do your Army-X-soldiers feel pain? If so, make sure they unterstand that you can make their immortal life very, very unpleasant. That might make them think twice.

Play bagpipes.

Nobody likes the sound of bagpipes. Threaten them to play for as long as the battle lasts. They will either kill you (and the bagpiper) on the spot, ending the problem for you immediately, or realize that immortality is really a bad idea when you have to spend a lifetime near a bagpipe player.

Of course you could always accept the fact that immortal armies are boring. But that would eb a different story.

Answer 15

There are a few ways to defeat such an army.

What you're basically saying is they're unkillable and can teleport out of situation they don't want to be in.

Here's two ways:

Sleeping darts. Capture them with sleeping darts/gas whatever and then keep them in a perpetual coma. Wouldn't it be cool if the Terracotta Army was actually an unkillable army, put into a coma only to be found 100s of years later? What if they wake up in the modern days?

Drugs. Get the opposing army addicted to drugs/narcotics you only own so their addiction overcomes the control their master has and they work for you in exchange for the next "shot".

Answer 16

Get them Drunk!

Not really a way to defeat them, but at least keep them immobile for a while, would be to spike their drinks/water with alcohol. Granted, the amount of alcohol that would be required to keep them blacked out drunk for an extended length of time would be quite large, but this would fulfill all the requirements for keeping them out of the fight, yet unable to transfer their bodies. When they are very inebriated, transferring their consciousness would be something beyond their mental state and could potentially reduce them to stumbling, easy'ish to control sacks of meat.

A good by-product of this would be that it might lead to some infighting between the immortal soldiers, however once dead, they would be sober again, so this would be a continual effort to get/keep them drunk.

This could also be done by challenging them to a drinking contest. A few of your soldiers getting alcohol poisoning, is a small price to pay.

Answer 17

I think the drugging/gassing solution is the best. After that here are a few alternatives.

Starvation. Burn everything edible, and poison every well within X (10-30) many days from the healer’s base. So they die of dehydration before they get anywhere, and get resurrected back in the center of the poison desert.

Fill in the suit of armor. (Concrete)

Throw the armor into the ocean.

Capture their family or those they care about.

Build impregnable defenses.

Build a defense that one person can hold.

Be nomadic so the enemy can’t get you.

Deploy Mongolian horse carvery. (Better bows, with longer range on better horses with more stamina, with better riders).

Answer 18

You can't without losing the battleground. You have to make the battleground lethal through chemicals or radiation (anything that produces continuing lethal damage). That way they can die and respawn continuously. Eventually, the enemy will stop sending troops into your area.

So, they would just go around unless you completely surround your self with a thick band of lethal land.

I don't know if I would consider that a win.

Answer 19

Since you did not specify them being immune to poison gas that is clearly the way to go. I mean even if they resurrect instantly the poison does not go anywhere and they will just cycle between death and resurrection.

Start by tricking them into a valley. This will contain the gas and make sustaining the effect much easier. The best is if you can make them camp for the night there. Treachery is your friend when dealing with immortals.

Opiates or other narcotics. You want them to be too confused to opt for resurrection to an armor outside the effect. Drugging food and water is probably the best as burning gives a distinct smell they might notice. You do not want to kill them just stop them from noticing the actual poison.

Carbon monoxide. It is a "colorless, odorless, and tasteless gas that is slightly less dense than air". In other words it is exactly what we want. And you can produce it simply by burning things with insufficient air, no advanced technology or science required.

While the immortals will revive after carbon monoxide kills them, they will not resurrect any wiser than they died. If you can pump the carbon monoxide level high enough which by the way also implies low oxygen level, they will not have time to understand what is going on and decide to revive elsewhere. They will be very confused. And they cannot detect what is killing them.

For that matter I would not be surprised if the revive was not fully effective in this case. It seems reasonable that it will fix the damage caused by poisoning but does it replenish the amount of oxygen in your blood? Or actually remove the poison? I doubt it does. It is mentioned they eat and drink and why would they do that if the magic sustaining them could replenish their energy, water, and oxygen?

This will work much better if you can either trick them to buildings that can be made air tight or can move them into such using golems or magically protected workers. I mean eventually you will have a storm with winds strong enough to clear the poison. You can of course protect the entire area with a magical field.

Answer 20

With all those rules, I thought of torture, since that doesn't kill them, and saps their will to fight... but that would classify as being stuck, and they could just abandon their bodies and take up a suit of the armour.

However, if they are rendered unconscious and are kept that way with a bit of primitive brain surgery, they will neither be dead nor stuck in a situation from which they have the mental capacity to desire escape.

It may cost the lives of hundreds to capture each immortal, but while troop quality is important, it can be trumped by quantity. Then, the immortals are rendered unconscious and made to be incapable of desiring escape yet not dead, and are kept alive until they die of old age. Faced with an army that is dwindling yet the soldiers don't come back either - neither dead or desirous of escape - the immortals will find themselves becoming increasingly irrelevant.

Answer 21

First of all, a terminology correction: Someone who exclusively fights people of vastly inferior skill, using lethal force against people who have absolutely no hope of retaliating in kind, is in no way, shape or form a hero.

They're bullies at best, and likely cowards -- it's no feat of courage to fear people who can't kill you or even permanently harm you.

Second, you're doing an amazing job of railroading every possible way that they could lose away, but that's your decision to make, so I'll point at the loophole you missed:

"You'd be amazed at what you can live through..."

If you're up against a force of unkillable bullies, and Option Zero ("Don't have a battle") isn't available, what you're going to have to do is throw out the entire rulebook about Laws of War, Rules of Engagement, and especially the parts about 'cruel and unusual', 'inhumane' and 'war crimes'.

They can't die, but they can still hurt, and they do still have a morale that can break.

Go all out VietCong -- booby traps under every bush, aimed to hurt and maim rather than kill. Sure, they'll be back momentarily, but if enough of them keep dying in screaming agony before reviving then sooner or later they'll want to pick a fight with someone who 'fights fair' -- meaning a fight they don't risk losing -- instead.

Answer 22

Cleansing fire

Immortal soldiers who spring back up from the grave, wounds fully healed, are a very difficult problem. As they are also 'heroic', they can kill a slew of 'normal' soldiers if they try to swarm them and just keep stabbing the immortal to death. You need something better.

Fire is the answer. It is low tech, incredibly painful and relatively easy to administer. The pain should in general also take the one on fire out of the battle. It leaves nasty wounds and if hot enough the skin itself can burn.

This last bit is important. Make sure the person is set on fire hot enough that they keep burning. They die. They revive. The fire has again fuel to keep on burning. Basically this cycle of burning and dying repeats until cooled off enough, oxygen has been taken away or the soldier stops reviving and goes to the armor.

How to do it? Many nations would have access to torches and flaming arrows. This in itself is already good, but the presents of torches and flame arrows often means oil as well. More refined oils can be created, burning hotter and brighter, but might not be needed. Now that you've got something that can burn hot and bright, you start chucking it against the enemy. This can be done like Molotov cocktails, or just throwing oil over the enemy and then lighting them on fire with torches. Other advantages are that you don't need direct hits, as splashes can already throw the (burning) oil against the enemy. Glancing hits are also very good.

If there is any doubt that they will keep burning, you just keep throwing on more oil. Once the burning death cycle starts, it'll be relatively easy to keep administering oil and it is very likely few immortals want to come back more than once. It is insanely painful. If they do, they might get mad and lose usefulness to the immortal empire.

This will cost a ton of manpower. Oil in those times is difficult to produce, as well as torches and the like. In battle many will still die, as the immortals are heroic people and being send against them is still a death sentence for many. But this way they have a chance. You'll likely support your oil throwers with a double line of long spear men, holding the heroes at a long distance and buying time to raise hell. Behind the oil throwers you'll have bowmen with fire arrows to just add to the fire for all oil that isn't ignited yet. The deaths that they cause can also help with more time, as well as speeding up the fire/death cycle.

The suits of armor require a different approach, but as they seem to be unable to resurrect a simple overpowering force is all that is required, using numbers and brute force to overpower them and then hammer them into unusable shapes.

Answer 23

I think it'd have to be a propaganda war, in the hopes you can get some of the immortals to turn traitor. Somewhat cliché, I know, but it's the most likely way of defeating them... surely all the immortal soldiers aren't thrilled at massacring defenseless folk, so you'd have to use that against them somehow.

As bad as it is, sending out child soldiers might be their best bet, that is bound to cause some dissention in the enemy ranks - unless they're essentially drones, but I didn't see mention of that in the question so I'm assuming otherwise.

It sounds like their only flaw is their immoral actions, so that would be the best place to focus your strategy, making the soldiers themselves realise that this is the case, otherwise I think you're just banging your head against a wall.

Answer 24

Your best option is to take out the ruler of this army. Create strife and disorder. These are soldiers, they follow orders. If no one is ordering them around, they might just go home. The best case scenario is a civil war. It would literally go on forever, or until they died of age. Problem solved. Another way would be to drive them insane, and release them. They would not re spawn, because they are still alive, but nor would they be a threat.

Answer 25

Well, since you can't really defeat per se your best bet would probably be slow it down, and incapacitate in some way. Someone already mentioned various sleeping gasses, but if this is Medieval level technology and those aren't available you could go for spikes on the ground, leg hold traps, snares etc.

For a more long term solution one may try to find some terrain that is especially easy to defend. I don't know what your world is like, but if there is a way to say, go across a massive river and destroy the bridges behind you, you can then leave a bunch of forces watching the river, whose task isn't to kill the enemy solders, but to thwart their attempts to rebuild the bridge. Or hole up somewhere in the mountains, where you can keep throwing rocks down to keep knocking the enemy down, etc. Basically any terrain that is difficult to get through and where being a strong warrior isn't really relevant.

Based on the comments it seems like they will respawn in the home base, inside a specially created armor if the body is destroyed, which does provide some options. For example, if you can lure them into some narrow area (Castle entrance or such) and then pour in some burning oil you can send a lot of them back at once. They'll respawn, but it will take them some time to get back and it might put a strain on their special-armor-production facilities, whatever they are. In any case you buy yourself some time to evacuate/hide, setup more traps, etc.

Ultimately, given the setup the most rational course of action would be to find methods to delay the enemy while you escape, since you can't really beat them

Answer 26

Feint

The number of soldiers in Army X is finite. Stonewall Jackson kept an army of 50 thousand invaders busy chasing him in the mountains (and essentially neutralized because they were far from the real fighting) by threatening the capital with, initially only 5 thousand of his own men in the Shenandoah Valley Campaign.

The weakness of Army X being exploited here is that they are: finite, and still only have limited knowledge of where and when the enemy is attacking. For all they know, the force exploiting the mountain pass to attack the Army X capitol really is the main force.

Overbear

Immortality does not increase the force multiplier for Army X to $ 1 : \infty $. Let's look at the guaranteed takedown case for any one soldier in Army X. One "friendly" soldier can die on Army X's blades, while 3 or 4 peers knockdown and tie up the Army X soldier. So, the force multiplier is $1:5$. A defender with 5 times as many soldiers as Army X in a particular field engagement has a reasonable expectation of winning at 20% loss of life. There's usually a rule-of-thumb of $1:3$ for defenders, so I believe these stack such that a non-Army X attacker will probably need 15 times the number of troops to win against dug-in Army X positions. But it is still a feat that can be done.

True, Army X can re-spawn at the Army X capitol, but it takes some amount of time to get back to the front. They are beaten for this engagement.

Defeat in Detail

Every successful General from Alexander the Great to Vice Admiral Hiroki Abe uses a combination of feint and overbear to take apart armies many times their size, by choosing to attack the enemy where the attacker possesses a numerical advantage. This is called Defeat in Detail.

We've determined that against unentrenched Army X targets, a force of 5000 ordinary soldiers attacking Army X scout group of 500 has every reason to expect victory, for a loss on their side of about 500. This can be repeated several times, taking down an Army X force expeditionary force of 2,500 if done in five 500-man chunks. This can scale to any arbitrary size with reinforcements among the ordinary soldiers.

The weakness being exploited here is that Army X soldiers are merely immortal, not all-strong.

Nets

Nets are a staple weapon in ancient armies, and will be very effective against Army X. The net comes in many form factors, but the one I imagine that would be most successful is metal links and lead weights and an attached line. It is a team weapon : one or two soldiers throw the net on a target and the rest of the team pulls the line to take the target off his feet, tighten the envelope, and tangle the target. This weapon would be effective even if Army X also had “the strength of 5 men”, because a solid metal link net requires a lot more strength than even that to break out of.