The crusades were in every way a defensive war. They were the West's belated response to the Muslim conquest of fully two-thirds of the Christian world. While the Arabs were busy in the seventh through the tenth centuries winning an opulent and sophisticated empire, Europe was defending itself against outside invaders and then digging out from the mess they left behind. Only in the eleventh century were Europeans able to take much notice of the East. The event that led to the crusades was the Turkish conquest of most of Christian Asia Minor (modern Turkey). The Christian emperor in Constantinople, faced with the loss of half of his empire, appealed for help to the rude but energetic Europeans. He got it.

Source:
http://www.crusades-encyclopedia.com/apologyforthecrusades.html

http://www.crusades-encyclopedia.com/crusadesmyths.html