Ever notice how theocracies with identical faiths don't attack each other, and those of different faiths, even of faint distinctions (eg Shia vs. Sunni), find war irresistible?
I think religious aggression is rooted in doubt. The mere existence of a culture that doesn't agree with your belief in your imaginary friend is volatile because it tickles the doubt that your imaginary friend
is imaginary. The more absurd the faith, the more aggression is engendered by doubt. (E.g.
the creationist who killed the evolutionist.)
Theists can be like 3 year olds who explode in rage when told St. Nick does not exist, because both God and Santa believers know, deep down, their imaginary papa may very well be a hoax.
Atheists are completely lacking in this incentive to go to war. Tell a Muslim that Allah is imaginary, and you may get a threat of beheading. Tell an atheist that Allah exists, and he'll ask for proof. Ergo, atheists are less likely to be incited to war over religion.
But the real reasons wars are declared (theft) have nothing to do with why they are fought (faith):
"
All wars are wars among thieves who are too cowardly to fight and who therefore induce the young manhood of the whole world to do the fighting for them." (Emma Goldman, 1917)
"Religion is regarded by the common people as true, by the wise as false, and
by rulers as useful." (Seneca, ca. 4 BC – AD 65)