There have been wars in which religious motivation has been a factor, although I think a lot of these things are political with a religious justification. For example, the Irish don’t like the British presence in their country, and probably wouldn’t even if there were no religious differences between Britain and Ireland.
Non-religious and even anti-religious states, such as the Soviet Union, have not made the world more peaceful. If some political leader were to decide that what was holding the human race back was religious belief, I could imagine them starting a war to attack religion. The only countries that have been officially atheistic have been communist countries, and they have a bad track record.