Population pressure does. Wars are ALWAYS robbery writ large. Basically its one group saying "Hey, they have stuff, lets take it" and then attempting to do just that. Even the Crusades were about securing wealth for Europe. Sure, people will use religion as an excuse. They also use political/economic systems, what color the skin is on one side or another, etc... as excuses. You don't see a call to ban politics, economics or skin color... So, lets stop being fools and attributing anything to a concept we invented.