a renewal of ecclesiastic influence. Monasteries, religious orders, and the Papacy itself were reformed. This renewal of Christian enthusiasm, however, relied heavily on the emergence of a powerful common enemy: Islam. A papal call to retake the city of Jerusalem led to the Crusades, and nearly two centuries of warfare between Christians and Muslims. The Crusades initially met with success, and a number of small, Christian kingdoms were established in Muslim territory. These, however, were short-lived, and the sacking of Constantinople by the Crusaders led to long-term animosity between the eastern and western branches of Christianity.