Another argument from Pinker on why war is in decline:
For one thing, it no longer pays. For centuries, wars reallocated huge territories, as empires were agglomerated or dismantled and states wiped off the map. But since shortly after World War II, virtually no borders have changed by force, and no member of the United Nations has disappeared through conquest. The Korean War caused a million deaths, but the border ended up where it started. The Iran-Iraq War killed 650,000 with the same result. Iraq's annexation of Kuwait in 1990 backfired. Israel seized land in 1967, but since then most has been returned and the rest remains contested.