You know why we have wars? We put forward a lot of pretexts, but the real reason is that people are and always will be asshats. It doesn't matter what creed they follow or don't, whatever they claim. The root cause is asshattery.
Sometimes an asshat invades Poland and a number of other countries. Sometimes an asshat wants to build a summer home on the Jordan. Sometimes an asshat shells Fort Sumter. Sometimes an asshat thinks, "Hey, Indochina went super well for France, let's try that too." Sometimes an asshat considers that Saladin looked at him funny. Sometimes, or in fact frequently, an asshat decides that other people are the wrong color. Sometimes an asshat crosses the Rubicon.
Everyone who starts a war flies the asshat banner ahead of his own flag, whether that flag bears cross, lilies, lions, crescent, or swastika. Or flying spaghetti monster.
NB Sometimes one must fight: when the question is, "Xerxes has crossed the Hellespont, what is your next move?" the answer is, in fact, "War."