Sixth Crusade

Sixth Crusade, 1227 - 1229

Upon the failure of the Fifth Crusade, when the victory had been so nearly within reach, the call to crusade began to go forth once again. Since Emperor Frederick II had failed to fulfill his vow in the Fifth Crusade, he came under papal pressure to do so now.

At the Council of Ferentino in 1223, Frederick renewed his Crusader vow in the presence of the Pope, the King of Jerusalem (John of Brienne, at the time), and the Grand Masters of the Military Orders.
The Crusades, by Dr. E.L. Skip Knox; The Sixth Crusade, p. 3, What's Another Vow, Among Friends?

... Holy Roman Emperor Frederick II vowed to lead a Crusade in 1215, but for domestic political reasons postponed his departure. Under pressure from Pope Gregory IX, Frederick and his army finally sailed from Italy in August 1227 ... .
Microsoft Encarta 98 Encyclopedia, Crusades; © 1993-1997 Microsoft Corporation. All rights reserved

Within a few days of his 1227 departure, Frederick fell ill, and was returned to port to heal, while his army continued without him. When he sent word to Gregory, Gregory excommunicated him. During his recovery, Frederick received envoys from the Sultan, Malik-al-Kameel, expressing his desire to negotiate. Upon his recovery, Frederick raised a second army and, in June 1228, set sail a second time, though still under the ban of excommunication. In 1229 he was able to, through diplomacy, arrive at a treaty with the Sultan by which Jerusalem, Bethlehem, Nazareth, and a corridor to the sea at Acre were given to the Christians, except that the Muslims maintained control of the Dome of the Rock, and the Al-Aqsa Mosque, as well as free access to them. In the meantime, Gregory had raised a papal army and launched an attack on Frederick's Kingdom of Sicily, which army was defeated upon the return of Frederick:

... Gregory was incensed at Frederick's presumption in leading a crusade while under ban of excommunication. Claiming provocation by Frederick's vicar in the Kingdom of Sicily, Gregory raised an army and launched an attack on the kingdom. This war marked the end of the policy of negotiation. Though Frederick's return witnessed the defeat of the papal forces ... .
Copyright © 1994-2000 Encyclopædia Britannica, Inc., Gregory IX

By virtue of the 1229 treaty, Jerusalem remained under the control of the Christians until 1244, when it was retaken by a Muslim alliance of the Khwarezmian Turks and the Egyptians. This led to the call for a Seventh Crusade.

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