This may be an over simplification, but there is war because there are ambitious people who hunger for wealth or power. There are plenty of people who want to live, period. They care not who rules them, or what conditions prevail, they only want to live. There are also people who want to live their lives as they choose, rather than be ruled by those power-hungry people.

When they meet, there is war.
I know some will disagree, but people and animals have more in common than some care to admit. There are ALPHA MALES in the animal and human worlds. However, in the animal world, seldom does the competetion for dominance over the pack, herd, or whatever result in the death of the losers. The ALPHA MALE wolf nips the loser on the throat, and the loser knows his place. In the human world when Hitler meets Chamberlain, Chamberlain, rolls on his back and exposes his throat, in an act of submission. Churchill, on the other hand, chooses to fight.
The bully does not target the strong, but the weak. Lions do not attack the ALPHA MALE Wildebeest, that would be tantamount to suicide; they attack the stragglers because isolated from the herd, they are weak. If the Wildebeest were to evolve any degree of intelligence, they would all run toward the attacking lions, rather than away from them. Lions would then be the ones that flee.