War has been passed down many generations. It has been of hatred and greediness.
When a government/president is not satisfied with their amount of land, they declare war with others. Of course, they get items from their war. Mostly the things governments/presidents want are land, weapons and resources.
When a rich country declares war with a poorer country, it doesn’t really end well. The poor country will have no resources and due to the damages of their buildings, most will be homeless. Also, they won’t have any weapons to defend themselves so they’d lose the war.
Sometimes it has a rare chance to have a Civil War. Civil wars happen when the same country but different parts want something from the other group. When the group/part doesn’t agree they usually declare a war.