What would a medieval war against a subterranean race look like?

To round off the little series started by this and this, I figured that I should add this question.

The setting: there is a race of underground people living beneath a human kingdom.

To avoid the connotations that come with ‘dwarf’, lets refer to these people as ‘molemen’ instead.

These molemen are short, stout, nearly blind, and are excellent burrowers and miners. They have evolved in an ecosystem that is completely removed the sun, their culture has no concept of the world above. They live in a vast network of deep tunnels and in extremely close-knit communities, and they have a very well-structured society.

These molemen farm lichen and mushrooms, they tender rats as livestock, and they gather roots. They harness heat from deep volcanic vents, and they draw water from underground aquifers. They are extremely proficient in tool-making and metallurgy - much more advanced than their counterparts on the surface. However, their numbers are smaller and their society is very rigid. Food is constantly limited, so the molemen live frugally.

The molemen have developed gunpowder, while the humans have not. They haven’t yet invented the musket, but the molemen are using fire lances similar to early China, as well as explosives used in mining.

For centuries, neither moleman nor human have had any knowledge of the other. The molemen shun the surface and tell horror stories about a ‘big ball of fire’ that hangs in an abyss. The humans have occasionally seen evidence of molemen (their sewage and waste expelled to the surface, for example) but have assumed that it's just some freak natural occurrence.

But then, this all changes. One of the molemen’s main tunnels is overmined and becomes structurally unstable. Above, the human’s capital city grows too large and heavy. After a series of earthquakes and tremors, the ground collapses - swallowing half the city into a pothole two hundred foot deep.

The two species are revealed to each other, and both their reactions are “Holy shit, there’s a bunch of monsters living down/up there!”

After this unpleasant first contact, conflict arises. Despite their aversion towards light, the molemen are drawn to the surface for its abundant food and supplies. The humans are drawn underground by rich veins of metal and gold.

War is declared, and the human king is very eager to crush these strange underground people.

But how?

As his advisors warn, the molemen’s tunnels are labyrinthine and they have no idea how far they stretch. It is very likely that there are tunnels beneath other cities and castles too, and there is a very serious risk of their infrastructure being undermined and collapsed from below.

Any regiments of soldiers sent below ground into the opening are at a severe disadvantage, being blind and lost. The molemen can easily ambush the soldiers in their tunnels, or collapse the tunnel and leave the soldiers trapped. The molemen have the superior weaponry, knowledge of the tunnels, and their underground defences are absolute.

Nobody knows how deep the tunnels go, but it is suspected to be hundreds of feet. The molemen are by the far better miners, and they are also fond of burying caches of gunpowder as explosive traps.

Even worse, the molemen can pop up from their tunnels anywhere, at any time, and there is no means of tracking their movements from the surface. The sun hurts their weak eyes, so the molemen often attack the surface at night.

The humans fear the molemen will collapse and overrun their nation from below.

They require a military strategy to both defend themselves and counter-attack, but how?

For answers, assume that this is a low-fantasy setting. Solutions that rely on magic are possible, but discouraged.

5 Answers
5

you don't need to fight them at all

turn them into the next Atlantis

if the humans city is near a river/sea (i would hope that it is) make the humans dig a trench over to that big gaping hole that just opened up, then just let the water do the work for you. The mole-men nation will all drown before they had a chance to fight back

To quote Pennywise

They all float down here.

Smoke them out

Another way without sending any men down there is to set fires in every tunnel entrance. The smoke will suffocate any mole-men near, making it easier to clear entire burrows in one go. Or drive the mole men to come to you where you surface dwellers will make quick work of them. Adding toxins to the fire will aid you in the killings

Poisoning their water supply

Even being a subterranean race they will have a water source somewhere. Poison the water and soon the mole-men will know why they fear the surface world. Even if all the water is underground there are ways to poison it, send a few brave men to the water, dig till you get to it ect.

We can eradicate the moles the dame eay that we have been eradicating other species for millenia - by destroying their ecossystem.

These molemen farm lichen and mushrooms, they tender rats as livestock, and they gather roots.

It is unrealistic for them to feed on lichen. Lichen is a symbioses between a fungus and photosynthetizing bacteria. That requires sun.

Shrooms depend on organic matter to decompose. Once it becomes the sole food source for them, they will be feeding their dead to it.

Roots - now this is where it's at. You can cause a lot of damage by finding out the plants from which the molemen gather roots, and watering those plants with lead or arsenic. Or you can just burn the upper portion of the plants to the ground - after all, the photosynthetizing that makes the roots so nutritious happens on the sun-exposed side.

The humans have occasionally seen evidence of molemen (their sewage and waste expelled to the surface, for example) but have assumed that it's just some freak natural occurrence.

I used to play and old PC game called Sim Ant, from the same minds behind Sim City. You control a nest of black ants and you have to destroy a nest of red ants. The game usually involves a lot of strategy regarding food acquisition, breeding a large amount of workes and soldiers, cornering the enemy and doing tunnel warfare to kill their queen.

But I discovered that you can cheat your way to victory quite fast by sealing the enemy's tunnels.

We are already poisoning the moles' food supply from above. Next, seal their waste and sewage tunnels. The smell inside should become unbearable to them.

Now, they can pop up from anywhere. But they can't farm anywhere, because they fear the very source of energy for practically every ecossystem on Earth. When they are feeding their dead to their shrooms, they will attack humanity to steal food. What we do then is guard our granaries. Use underground traps - let them dig through a cesspit or tar if they want to come from below. Otherwise they have to come marching, and that's warfare that we are used to.

When they are so poisoned, starved and covered in waste that they can't go on anymore, they will either give up or move somewhere else. We win.

1) Force disorganization

Find an entrance tunnel of theirs and dump some toxic gas (smoke works), preferably denser than air. The gas sinks into their deepest tunnels. This force the moles to rearrange, as their most valuable resources will most likely be down there, including their children. Continue doing this, until a significant portion of their tunnels is filled with the gas. Smoke also works for this purpose.

Alternatively, if there is a large body of water nearby, make a connection from their tunnels to it. This may require the deaths of a few workers, but it will surely force the molemen out of their deepest tunnels. The body of water just needs to be close to one of their waste tunnels, because there must be other tunnels near there.

2) Attack

The molemen are disorganized. A significant portion of their manpower is devoted to making their tunnels less accessible to the human chemicals. Send men with a light source to charge through the tunnels. The news of the flood will have reached all the soldiers, making them panic, and more likely to flee. A couple groups of your soldiers might die from the moles, but you should make most flee.

To create even more confusion, in the previous step, pour oil into the tunnels of the molemen to make them think that the fireball in the abyss is spending into their tunnels.

Your soldiers will follow the molemen through the tunnels, hopefully leading them into a "city" of tunnels. Your soldiers should then massacre all of the mole men they find. Follow the molemen that are fleeing to the next city. Repeat.

Use the natives against the natives.

When asked by George Marshall in 1942 how the Army should train for
pivoting from the war in Europe to the Pacific, the commander of the
Marines on Guadalcanal answered “go back to the tactics of the French
and Indian days . . . study their tactics and fit in our modern
weapons, and you have a solution.”
https://www.hoover.org/research/lessons-indian-wars

Fighting strange natives on their home turf is a risky endeavor. But maybe these warlike natives like to fight each other? Co-opt one group of natives with your resources and wealth and then let them teach you how to fight their own kind. This is what the Americans did with the Indians during the settling of North America.

So too with these molemen. Fighting them all in the tunnels is a losing proposition. The king will need to learn their ways and then back one faction. Toplander food alone will give that moleman faction an advantage. If the molemen do not have liquor, see how they like that too. Then have your allied molemen help you wipe out the rest or drive them deeper underground, giving you access to the mines.

Let the germs do the work.

Move quickly in the above endeavor. The molemen might not have been exposed to surface world diseases and you might find your allies getting sick. Disease actually did much of the heavy lifting softening up the Amerinds for conquest. It would be good to learn your way around down there before they all die. Note: disease can be a two edged sword. The Europeans had smallpox and measles to share but the Amerinds handed off the Great Pox syphilis. The molemen might have diseases of their own your toplanders have never seen.

Moles rely in their highly developed hearing for probing their surrounding environment. It's highly likely that also your molemen will do the same.

Therefore your only hope for limiting them is to create loud noises underground. Lacking explosives, you have to rely on mechanical means: a series of trip hammers constantly hitting poles in the ground would create a loud underground environment, ensuring a molemen free zone.

This could ensure some protection to location like mines, wells or deep cellars. I don't think the molemen are a real threat for cities and buildings: since you mention they are afraid of the outer environment, they will probably refrain from being accidentally exposed to it.

You might even try using molemen-proof poles (if existing) to create a sort of underground maze to hamper their movement.

But that apart I don't think you have real means of attacking them.

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