it was a super-tank, don't fuck with super-tanks, and the point of it all was to stop the war from happening, the world as it was, it was already falling apart, the scene where they were on a battlefield (in a flashback) where the captain fell into a hole, it was the top floor of an old skyscraper, that might give you an idea of how old that world became, and what happeend, it was all a disaster in the past, lucky they didn't get off with nukes, the girls were ready to sacrifice themselves to stop a meaningless war.