If you read Clauswitz, then you know that war, or armed force, is simply a tool that a nation state can use to achieve its goals and objectives, along with trade, diplomacy and espionage. That would allow for Greed (Opium War and Crusade IV), Stupidity (WWI), Religion (Crusades I, II and III and the expansionist wars of Islam) and the every popular Quest for Power (Franco-Prussian War or Austrian-Prussian War and WWII) as reasons.
Another line of thought indicates that overpopulation, especially of young males, causes wars. I think this is a contributing, or enabling, factor, but not a cause by itself.