An illustration of crusaders before Jerusalem. The Crusades were a series of military campaigns of a religious character waged by much of Christian Europe against external and internal opponents. Crusades were fought mainly against Muslims, though campaigns were also directed against pagan Slavs, Jews, Russian and Greek Orthodox Christians, Mongols, Cathars, Hussites, Waldensians, Old Prussians and political enemies of the popes. Crusaders took vows and were granted an indulgence for past sins. The Crusades originally had the goal of recapturing Jerusalem and the Holy Land from Muslim rule and were launched in response to a call from the Eastern Orthodox Byzantine Empire for help against the expansion of the Muslim Seljuk Turks into Anatolia.
William Swinton Outlines of the Worlds History, Ancient Mediabal and Mondern, with special relation to the history of civilization and the progress of mankind (New York, NY: Ivison, Blakeman and Company, 1874)