I don't think the "religion causes wars" argument is put by atheists nearly as often as religionists would have us believe. But if you do actually encounter it, then yes, you're entitled to point out another cause of war and ask what is the correct attitude to that.
I fear that you're right about people's "moral" choices depending on their own circumstances - not everybody, of course, but a pretty large proportion. Certainly it's a lot easier to take a high moral tone if it's not costing you anything.
Maybe that's why religionists tell us we can't be trusted to make our own moral choices and have to take their word for what a god wants us to do.
But seeing as the directives of any organised religion or secct thereof tend to change over time and from place to place, it seems to me that to take your moral stance from a church of any kind is no different from just acting according to the expectations of non-religious society. Hmm...