The First Crusade was initiated in 1095 by Pope Urban II. The purpose was to assist the Byzantine empire in reclaiming the Holy Land from the Muslims even though the Byzantine Empire had been estranged from Rome for many years. The First Crusade was mostly successful but instead of the Holy Land being returned to the Byzantine Empire four separate crusader states where created. Following crusades were sent for a variety of reasons. Some were created to assist the Byzantine Empire and the newly created states in the Holy land. Some were sent to overthrow the Byzantine Empire. Later Crusades were sent to Spain to help fight the Muslims in that country. Some Crusades were sent to fight pagans in eastern Europe and then some Crusades were sent to fight Christian countries in Europe that the Pope didn’t like.

(The Great Courses – How the Crusades Changed History)