If there is God ,why do war happen?Watch
Some people believe that it's a righteous thing to "bring judgement" to people who don't believe with their God's support.
Also there's that argument about free will- God giving people the free will to do good or evil as they wish because if all people were just purely innocent god-praising robots, the idea of "good" would be kind of meaningless- which puts the blame for war on people choosing to be evil.
Though seeing as all people and things that exist were supposedly entirely created by God, surely that means the evil itself was in some way created by God...
Other arguments seem to go something on the lines of "God has some higher reason for allowing suffering and war that we humans can't possibly understand."
Some people believe that God is a hateful, tyrranical being who revels in war. In that case, wars happen because he wishes it to be. If that's true then we're all merely puppets. However, no one knows what God is or how hes like, or even if he exists. That's why religions are called faiths and not hard cold truth.
Whatever you believe in, as long as that's what you truly believe in, that's what matters.