Wars are started by people, often for a political motive.
Quite an empty argument. You can say that about all conflicts attributed to one cause or another. It's not oil, it's politics. It's not ethnic strife, it's politics. Homophobia in Uganda? Bad people, not bad religion. Vatican coverup of pedophile priests? Fallability of people, not the religion. Etc, etc.
Also, since religion and politics have essentially been the same or overlapped to a big degree, political motivations may well be the same as religious. I'd say that it's the case even today to some degree. It certainly seems that way in the US, and is obviously true for many middle eastern nations and groups.
I'd even say that the current "war on terror" is mostly a bunch of idiots who are trying to use the trappings of religion for political purposes, and it isn't the religion itself that is the motivating factor.
Obviously there are political motivations, but you don't have to look far on either side to find religious zealotry as a strong force, especially for those who do the fighting. Blaming only religion is simplistic, but so is not blaming religion at all.
Mao Zedong and Joseph Stalin, both professed atheists, can account for far more death, misery, wars, and famine than almost all religious leaders or even religiously inclined political leaders in the entire history of humanity combined.
Atheism is the lack of belief in gods. It's hard to argue that someone went to war or comitted genocide based on the lack of belief in something. No war I've ever heard of has been fought to spread a lack of belief.
The anti-religious dogma of communism isn't effectively different than the anti-religion dogma of religion towards other faiths—your beliefs bad, mine good. The lack of a supernatural god seems quite inconsequential seeing how god-like Mao and Stalin were in many respects, and how dogmatic their teachings were. North Korea is a good modern example of how this works. It's not because of atheism that North Koreans risk their lives to save a portrait of Dear Leader, it's because of the very dogmatic, unquestioning system that can make religion fuel wars and oppression.