Population Pressure and War
Population pressure appears to play a major role in Chimpanzee "warfare", and it might well play a similar role in human affairs. The turmoil in the Africa and the Middle East is associated with drought and population growth. The relative peace in Europe during the Nineteenth Century may well have been related to the opportunities of young men to emigrate to the Americas, Australia, and the colonies.
Historians ought to pay close attention to this sort of meta-factor.
How we fare in empires vs. independent nations is also worth a look.