Arthur Clarke used to tell anyone who would listen that Buddhism was the only religion that had never had a holy war. Then the Buddhists of Sri Lanka started massacring their Hindu neighbours. I sometimes wonder if that was what killed him.
But given the record of religious persecution in the Soviet Union and China, one certainly can't claim that atheists have never been guilty of religious war. And given the pogroms against the early Xians in Rome, we Pagans aren't guiltess either...
(I suddenly remember Pratchett's "all-purpose warcry", "Remember the atrocities committed against us last time which will justify the atrocities we're about to commit!")