Who, where, what, why etc vary according to how far down the rabbit hole they have gone.
In reality an underground facility is less exposed, is safer from general attack and when nuclear war was a genuine fear gave a level of continuity if it could be self sufficient - like the ‘secret nuclear bunkers’ that now have tourist road signs pointing towards them
A large complex is suitable for tunnels, perhaps a ‘safe’ route between related bunkers, but a tunnel complex of its own linking military bunkers is counter productive - you have secure separated bunkers safely isolated in case something happens to one, and then you connect them all together taking away the entire point
Whos fighting in them?
In a real ‘underground’ war large complexes would be sat at screens controlling missiles
In the extreme rabbit hole it’s entire army’s of underground infantry attacking each others bunkers (all squished up in a tunnel corridor ???)
What armies would they be?
Not the public overground governments army, but the other army of the secret government that really runs things behind the pretend public government
Why?
Who knows - the world has descended into sodom & Gomorrah, so the real battle is between the good vs the evil
Particularly the highly moral Trump and Putin on the side of perfect 1950s morals of a world that never existed against the evils of today
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