1) Greed
2) The guys in charge simply don't like the other country. North Korea would be an excellent example of the latter. I suspect there will eventually be a war with them, unless something big happens over there.
3) The guy in charge is insane/sadistic: i.e. Hitler, Stalin...Kim Jon Il/Un may fall into that category too.
4) Politics. Some may argue the wars in Vietnam and Korea were set up to make the U.S. look good. It may be argued as well that it was altruistic on their part, to get rid of the oppressive government. We'll never know for sure.
Edited by Chris Miller, 23 April 2013 - 12:18 PM.