Atheists often claim that religion fuels aggressive wars, both because it exacerbates antagonisms between opponents and also because it gives aggressors confidence by making them feel as if they have God on their side. Lots of wars certainly looks as if they are motivated by religion. Just think about conflicts in Northern Ireland, the Middle East, the Balkans, the Asian subcontinent, Indonesia, and various parts of Africa.
However, none of these wars is exclusively religious. They always involve political, economic, and ethnic disputes as well. That makes it hard to specify how much role, if any, religion itself had in causing any particular war. Defenders of religion argue that religious language is misused to justify what warmongers wanted to do independently of religion. This hypothesis might seem implausible to some, but it is hard to refute, partly because we do not have enough data points, and there is so much variation among wars. In any case, the high number of apparently religious wars at least suggests that secular societies are unlikely to be more prone to murder in war.
Morality Without God? (Oxford University Press: 2009).