It is interesting that both you and tarazkp decided to write about war today. I guess that shows that your minds are really similar.

In addition to humans being fucking stupid and greedy creatures I think that wars start as a dick army measuring contest. Every country wants to be stronger( or strongest) so they invest more and more into war machine. This means that other countries feel like they losing so they invest more as well. And so the race continues. Occasionally countries feels the need to show that their armies are needed to justify spending insane amounts on military. And what is better way to do that than attack your neighbor. Winners get to brag about their military being the stronger one. Losers see the need to catch up and spend more on military as well. And since politicians are not the ones being killed in front lines it is almost a win win situation. At least for those who have a deciding vote.