Warfare and political violence is, as Klausewitz postulated, the exertion of political will over another -- that is the end goal of what warfare is out for. But it's a tricky thing to pull off; Ending a war is a lot harder than starting a war, as it relies on your opponent's mercy, and they might not have much after you attacked them first.

A society continuously at war, however, finds it much easier to stay at war. You don't have to worry about who attacked who first -- you already have an enemy to contend with, and you'd have to trust for peace to happen.

Nations find it easy to go to war. Nation-states innately come with the idea "my nation is wonderful, theirs is not as wonderful; I don't want to be them, because I am X. I will fight for this." Nation-states involve a solidifying of identity and the encouragement of defending that identity to the death. While this offers self defense against other nation-states that think the same way, it's no defense for those in your nation that "don't belong" for its cohesion -- a big motivator for genocide (Armenian genocide, the Holocaust, etc.)

Restricting national identity leads to in/out groups

At the reverse, and something that concerns me, is that implies it's harder for people to unite around ideals devoid of nations.