The reasons for war have and haven't changed, same thing with the way wars are fought. It's still politics and land, but there are other factors I can't quite place. The weapons have changed quite a bit too.
However, war has changed because it's no longer as honerable or masculin as it used to be. I mean, --and many people are going to disagree with me on this-- war nowadays is just standing on the opposite side of a large space of ground from another guy, both of you have guns, and both a trying to shoot the other. People who go into war aren't nearly as brave as they used to. Back in the old days, and I mean the REALLY old days, people had to actually go up to you and bassically say "I'm going to stab you through the stomach" or "I will cut your head off" before actually doing so. Even archers were liable to get their legs cut off at the hip. Soldiers today are cowards becuase you have to make sure your enemy can't see you before striking to make sure everything goes well. It was never really like that in the middle ages. To fight a war back then, you took a few hundred people, gave them spears, organized them into lines, and said "Kill everything that is not wearing the same kind of armor as the people you see standing around you."
Take, for example, Japan. In mideval Japan, generals of warring armies sent each other gifts, such as poems or salt, so to honor each otehr before battle. Some of the time, battle would end the moment the gifts were delivered, and both generals would all off their troops.
Disagree with me all you like, war is endlessly more cowardly then it used to be.