Whether Hitler was an atheist or not is debatable, but what can be said pretty categorically is that he used faith in order to bolster his political ideology this is of course against the Jews and with the majority of the Catholic population of Germany at the time. Hitler spoke of "a God" often, both of the power of this being and its need to fight off the atheist oppressors of Soviet Russia - like I say whether or not he truly believed on one really doesn't matter, a deity was used in order to carry out and achieve a goal.
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So to say that he was fundamentally an atheist and his atheist mindset allowed him to achieve what he did is somewhat fallacious because it is not the public image he chose to give himself with when he needed to gain control of the masses. Of course this can all also be said of a number of leaders who knew the power of the faith their public held who would do anything for them and then used it to their own advantage politically.
But this really isn't the point, while it cannot be denied that religion can be a part of war and violence of which you could run off numerous examples it would be foolish to equate religion as the only cause. Power, land, natural resources and wealth are all major reasons for the push to war - there are others of course but at the moment this point really doesn't need further clarification. So in light of this and as the OP asks would there really be all that much difference between that world without religion and our own? We can speculate and list a few examples where bloodshed "might" not have been spilt but then again we have to think about where it could. Until we get over the mentality that our nations are our very own tribes then there really isn't going to be any less death in the world and even then whose to say there won't. Human nature is human nature, group think is just one small part of it.