Crusades

 In the late 11th century, Jerusalem was controlled by the Muslims. This is Islam's 3rd holiest city but it is also the Holy City to Jews and Christians. In 1093, Emperor Alexius Comnenus wrote a letter to Pope Urban II
encouraging the Christians to recapture the Holy City. This began 200 years of religious military campaigns. Crusade means "taking of the cross". "Soldiers of the Church" took vows to take back the Holy City and were promised immediate remission for their sins by the Pope. the Muslims end up fighting off the Christians after 200 years of back and forth fighting. Religious intolerance grows and Christians and Muslims increase the rift between them.