That's because most wars were fought when religion was very much a state issue. While politics could be the obvious reason, religion was very important. Look at renaissance/early modern Europe. The whole Mary, Queen of the Scots problem started mainly because she was Catholic; the ruling Protestant minority wanted Britain to stay a Protestant, despite much or most of the peple still being Catholic. One could say that modern wars are less about religion, but look at the rhetoric through the Cold War and most of the wars in the Middle East (including Iran-Iraq). Even now, look at such people as Ann Coulter or Eric Prince, and their 'crusade' against Islam. Even if politics is the most obvious problem, I can guarantee many people will see war as a religious issue (except Richelieu).
"•In the twentieth century, atheistic regimes killed three times more people in peacetime than those killed in all the wars and individual crimes combined"
This is from Irrational Atheist, and this gets my goat. The so-called communist, atheist regimes were not atheist. Stalin, Mao, Hitler, etc removed the established relgion, and then installed a new one, one based on themselves as god. The cults of personality were new religion. Like Robespierre installed a new calender to break down old traditions, so new gods of Hitler, Lenin, Stalin and Mao were installed to break down old traditions and mould society how they saw fit. Also, thats because in this century we have created much more efficient ways of killing people, if Europe had the same level of technology in the 1600s that we have now, imagine the Thirty Years War.
Anyway, what is the point of religion? I see no reason for God/dess/etc in my life.