Misunderstanding and conflict between the monotheistic faiths:
Monotheism is the belief that there is only one god and is opposite of the religion who believe in the existence of many gods like the religion of Judaism or Christianity.
Crusades were organised by the Christians who were in western part of Europe who wanted to stop the expansion of Muslim to their area.
They also wanted to regain the area of middle east and other territories where there was Christianity before the expansion of Muslim in that region to again spread Christianity.