The Campaign Builder's Guild

I just read a really nice campaign in the homebrew forum,  Shattered Kingdoms: The World of Karis, and it got me thinking about war. My own campaign timeline starts soon after a century + long war. How many of use war or the aftermath of war as a major element in our campaign worlds?

What is t about war that seems to make it so 'desirable' for a campaign world?

Because turmoil is left in the wake of war, and turmoil provides an excellent background for adventuring. Adventures don't happen as easily when everything is sweetness and light.

Pretty much what Gilladian said.  Also, an time of political tension is going to create opportunity and motivation for people to act to change things.  This could be the heroes or the villains, but in either case, one side can hope to benefit from (or improve) the situation.  War and its aftermath is obviously a time of great political tension.