What led to the crusades?

The crusades began when the Muslims stole the holy land of the Christians, Jerusalem. 3 main crusades were held to gain back the land. The christians had advance technologies and won the first crusade, but the Muslims quickly advanced their technologies to match those of the Christians and ended up loosing nothing. Slowly, however, the Muslim's declining society caused the Christians to slowly chip away at their land, until they once again were able to regain Jerusalem and a few other cities.