Those alliances stopped endless war. You can point to many causes, boiling it down to one is a degradation of history. For WW2, I believe it was Rubystars that pointed out that Nationalists love their country and only want their country, commies want to take over the world for their ideology. Loving your country doesn't destroy it and others, putting an ideology above simple needs makes a complicated honor system to adhere to an ideology. "Where honor is complicated, life is cheap", which was the conclusion to a study why Japanese people and Saudi women commit suicide the most.
You can't give a single reason for either war, the most honest attempt I've seen went even broader and said "evil begins where empathy ends".
Marcon is just saying, "people that oppose me destroying France with Muslims want to destroy the world like Nazis they're worse". Ironically, we see the flip side of the evils from lack of empathy, if you have none and judge good people as monsters, you can do monstrous evils to them, and if you have none and judge monsters as good people, you can stand and support the most monstrous evils.