Why on earth do you think WW1 is a good example?
Try to differentiate between correlation and causation by the way, the reason for violence occuring in monocultural societies isn't grounded in them being monocultural. Compare that to, say, the rotherham gangs etc, that entirely target their victims based on their skin color and culture.
Why the fuck would Sweden become safer by throwing in a couple of million somalis, arabs and afghanis?