Explain the Crusades
The Crusades were a series of events where the leaders of a religion (which believes there's an invisible Man up in the sky who makes you an offer you can't refuse) decided that going off to kill innocent people is a great idea and compatible with the faith.
It should seem very familiar to anyone in the modern age. Crusades then, crusades now.
"Innocent"? Not exactly. Muslims captured Jerusalem and sent their armies out into the world (Moors, mamluks, etc..) The claim that the crusades were nothing more than unprovoked rampaging and looting is muslim apologist bullshit. When one religious group attacks another's holy site and captures it, what do they expect to happen??
Many horrible things have been done in the name of every religion on Earth, but I tire of seeing the crusades constantly misrepresented as nothing more than a murdering/looting spree, and the false portrayal of muslims as their innocent victims.
If all else fails, lower your standards.