Two underground dwarven races are fighting a war. Dwarfs usually live in mines, but they can also move on surface. One army of dwarfs also control giants. The upper ground terrain is made of mountains that giants can climb. The technology available is Medieval. Giving those facts how would a war like this work? Are the giants reliable?
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No or very little surface warfare. There is no point in risking surface combat with giants, much safer would be focusing on underground warfare
Now, underground warfare would be very, very messy thing. Tunnels can be easily turned into choke points, it is easy to build fortified gatehouses, dig parallel tunnels with arrow slits and murder holes, build some sort of "drawing bridges", tunnel can be twisted to stop battering rams, etc... So, to avoid those choke points, attacker is forced to mine their own assault tunnels, bypassing fortified lines of defence and entering into enemy´s unprotected rear. But, defender is aware of this tactic and is trying to locate attacker´s miners and stop them before it´s too late. Well, that`s short describtion of mining warfare...
About actual hand to hand combat... There is no space for flanking maneuvers or usage of range units in tunnels. All dwarves can do is pushing forward. So, the tactics they can use is fighting in close formation similar to greek phalanx or roman maniple. The best weapons for this kind of combatis big shield and short stabbing weapons - spears or gladius style swords. Long pikes, polearms or slashing weapon are not good choice, because of conditios in tunnels.
Like any war: messy...
You've got all the troop level weapons available to you, plus on the surface you could still use siege weapons, canon etc against giants and mine entrances. I suppose you could have small dwarf-portable mortars underground for use in caverns on a low trajectory, but your side would need good earplugs.
There is history in underground warfare in Europe with Great War mining battalions, trying to lay huge charges under the enemy lines, and also in the tunnels in Vietnam.
Both fire and flooding are serious risks in a mine. Would setting these be considered a step too far in your dwarf culture? Would the use of smoke or noxious/poisonous fumes be considered bad form too given the risk to civilians/peasants, assuming your dwarves have a code of honour or a concept of chivalry. Or maybe more fun for them to be "murderous little bastards, especially when drunk".
Huge scope, sounds lime you could have a lot of fun with this.
Are the tunnels connected? Are those connections known?
There could be big, known tunnels with border checkpoints. In that case, combat could have some similarities with siege warfare.
- Defenders could put murder holes into the roof of the tunnel, to be reached from a higher gallery.
- There could be arrow slits, to be reached from a parallel tunnel.
- The tunnels would be twisted to stop battering rams, but that would also hinder peacetime commerce.
Or there are no known connections. Both sides would try to dig tunnels which open into each other's rear areas.
- Look into the original meaning of mine warfare.
- Plans of the enemy mine would be a great target for spies.
- Each side would try to listen to enemy miners, or check for vibrations. Dinner plates with water on the floor, to look for waves. That can be difficult if the own side has a bustling, noisy city.
- There might be listening periods at random times, communicated through the city. Town criers? "Silence in ten minutes! Silence in ten minutes! ..."
The dwarves don't like to go out. It is weird up there. But their dwarf hearts are full of hate for their brother dwarves and each dwarf clan likes to think the other group is suffering and dying because of their cursed differences.
The one group of dwarves has giants and so they send the giants to do their fighting. The other group has something else which is more or less evenly matched with giants - trolls. The trolls do the fighting for that side. Trolls and giants are supported with the labors of the dwarves, who provide supplies and weapons.
The trolls and giants report back to the dwarves about their victories and very occasional losses. Sometimes a troll comes back torn up, or a giant fails to return and is lost to the war. The battles are reportedly brutal and bloody. Each campaign brings its side closer to victory. Soon (very soon!) they will celebrate collapse and burning of the enemy tunnels, their inhabitants forced screaming into the light where they will be squashed like the squirming worms they are!