FROM THE FOURTH TO THE SIXTH CRUSADE 1229) Though the powerful Pope Innocent Ill called for a new crusade in 1198, power struggles in and between Europe and Byzantium drove the Crusaders to divert their mission in order to topple the reigning Byzantine emperor, Alexius III, in favor of his nephew, who became Alexius IV in mid-1203. The new emperor's attempts to submit the Byzantine church to Rome met with stiff resistance, and Alexius IV was strangled after a palace coup in early 1204. In response, the Crusaders declared war on Constantinople, and the Fourth Crusade ended with the conquest and looting of the magnificent Byzantine capital later that year The remainder of the 13th century saw a variety of Crusades aimed not so much at toppling Muslim forces in the Holy Land as at combating any and all of those seen as enemies of the Christian faith. The Albigensian crusade (1208-29) aimed to root out the heretical Cathari or Albigensian sect of Christianity in France, while the Baltic Crusades (1211-25) sought to subdue pagans in Transylvania. In Crusade, put in motion by Pope Innocent Ill the Fifth before his death 1216, the Crusaders attacked Egypt from both land and sea, but were in forced to surrender to Muslim defenders led by Saladin's nephew, Al-Malik al-Kamil, in 1221. In 1229, in what became known as the Sixth Crusade, Emperor Frederick Il achieved the peaceful transfer of Jerusalem to Crusader control through negotiation with al-Kamil. The peace treaty expired a decade later, and Muslims easily regained control of Jerusalem.