Religious warfare
It is interesting that ancient times seem to have less of what we see as religious warfare. However, if you expand the concept of religion a little, to where national leaders were actually considered gods themselves, than you can open up the field a little bit.
Egypt may not have gone to war for Anubis, but they sure as heck went to war for the Pharoah. Same for the emperors of the eastern civilizations. Caesars are probably stretching it a bit.
Still, going to war to kill and die for the Great Leader is, psychologically, no different than any other religious crusade.
Remember that most of the atrocities of the past century were from societies that considered themselves non-religious.