Today we hear about the Crusades being Christians vs Muslims, but what about the Christian-nations that sided with the Muslims? Why did they do so?

by Execute-Order-66
AlviseFalier

Which christian nations?

The crusades were by and large expeditions organized by single individuals, not entire states. Even when monarchs went on crusade, it seldom involved the mobilization of the entire country. I'm going to have to ask you have to clarify what you're talking about.