War

Why do wars start?  I understand some (or all) people can be angered to the point of wanting to physically harm someone else, but what leads to large scale wars?

For example:

Nation A has a philosophical disagreement with Nation B.  They have been arguing for 50 years about the right to paint cows blue, but there is no resolution.  Does it logically follow that Nation A should say "Let's beat an agreement out of them" and start a war?  

Obviously, most people would think this is a stupid topic to go to war for, but what is an appropriate reason to go to war for?  What is the breaking point where you have to turn to physical means because intellectual arguments can only get you so far?