The Crusades were a series of military campaigns undertaken by Western European countries between the 11th and 16th century. Originally called by the Pope of the Catholic Church, the stated first goal was to take up the cross and restore Christian control over the Holy Land (namely Jerusalem). In total Western Europe engaged in over 10 crusades, though not all were for this original purpose. At first the Crusades were fought to assist the Byzantine Empire, who requested European help in fighting off the expansion of the Muslim Turks. In time, though, the term crusade was used to describe wars against pagans, heretics, and those threatened with excommunication from the Church. Interesting, one set of Crusades actually led to an alliance between Christians and Muslims, and another was called against fellow Catholics.
Jonathan Howard looks into the history of the crusades in this short eBook.