Besides the A.I. being borked, I think I figured out why a military rampage goes unchecked, and it's not war weariness. It's the fact that they divided gold and science. Previously, in order to maintain units and newly gained cities, you had to tank your science rating in order to complete a war, eventually falling behind enough that you couldn't continue the war. Now your science increases as you fight, giving you stronger units to continue fighting.