I'm fairly certain humanity would find plenty of reasons to wage war if religions were not around to blame it on.
For sure, there's plenty examples of people of the same religion going to war over various reasons like land, resources, geopolitical reasons, wars of oppression, wars of liberation, power and control with wars of succession and so on. But for most of the genuine atrocities you need more, you need such a burning hate for the opposition that you're willing to slaughter down women and children and burn their cities to the ground. Where simply victory isn't enough, only total submission or even extermination. Religion is a very common fuel for such hate.
When fighting for resources you're also looking for a "return on investment", you have to gain enough to be worth going to war. That you can usually defend against by rational investments in defense, making it too costly to be worth it. Irrational wars fueled by hatred often don't care, a civil war might tear the whole country apart and leave it in ruins as long as the infidels die. The Germans fought the allies all the way back to Berlin, the Japanese until they were nuked. Twice. Neither made sense, it was death before surrender.
Of course you can say that was mostly racism, not religion though I'm pretty sure the Holocaust was a good dose of both though Nazi Germany certainly fought a lot of other nominally Christian nations. Religion is also a very lasting divide. Germany fought most of Europe, now they're a key member of the EU. The US was at war with Mexico, the North was at war with the South but the wounds mostly die with the generation that experienced it. In the middle east they've been fighting for 2000 years and every conflict reopens a wound that never heals.
Of course religion has its good sides, I think a lot of people behave better than they might have because they think God/Allah is watching or it affects their karma. So it's not just irrational evil, it's also irrational good. Mostly just irrational and mostly harmless, really I don't care if you want to bend knee and pray to the FSM or have your own diet because FSM said eating something is unclean or the FSM told you not to work on a Sunday or whatever. As long as you got it dialed down to mostly quaint and charming.