Vader and Murtaw have been at each other's throats for a long time at this point. Neither country communicates very well about [i]where[/i] they do their digging, so while there are borders in place, they aren't very well respected. A lot of the takeovers are a case of, "You built this on our territory, so it's [i]ours[/i]!" and, "[i]Nuh-uuuh[/i]! We built it, so it's [i]ours[/i]." Before all the fighting started, for a lot of people, it was just a cool thing to do to become a soldier, 'cause you got to carry a gun and brag about being a stone-cold killer. I mean, it's not like there was going to be a war or anything, right?

It doesn't help that, while tunneling and general excavation technology in general has advanced very far, population growth outstrips the rate of expansion, even when not counting immigrants. Also, a good chunk of the surface fauna is...not nice.