Probably not even religion, but anything that unites a tribe. Religion is useful if you're looking to invade a Muslim realm, but race, class/status, and political ideology are all popular rationalizations for motivating attrocities.

Religion is also a useful pretext if you are a merchant competing with Jewish merchants. Religion is real enough if you are promoting one religion as the only true way and there are people practicing another religion in the vicinity who seem to be going fine and undercut your claim of "the only true way." In that case you might find common cause with the merchants who want to get rid of the competition. Having an ignorant rabble hungry for loot at your disposal doesn't hurt either.

I think you may have missed my point which was that it’s not religion _per se_ which provides the pretext, but rather any appeal to our tribal nature (religion is a favorite, but any sufficiently strong tribal boundary will suffice—racism, nationalism, secularism/theism, political ideology, etc).