Sequence of events
Frederick II, Holy Roman Emperor led the Sixth Crusade to the Holy Land in 1228, and claimed the kingship of Jerusalem by right of his wife, Queen Yolande of Jerusalem, who had inherited the title of 'Queen of Jerusalem' from her mother, Maria of Montferrat, the wife of John of Brienne.
The size of Frederick II's army and his reputation was sufficient to regain Jerusalem, Bethlehem, Nazareth and a number of neighbouring castles by treaty from the Ayyubid Sultan Al-Kamil. However, Jerusalem did not remain for long in Christian hands, as there was not enough Christian-held hinterland to make it defensible.
The Ayyubids invited the free-roaming Khwarazmian clans, whose empire had been destroyed by the Mongols, to reconquer the city. In the siege and subsequent fall of the city in July, 1244, the Khwarezmians completely razed Jerusalem, leaving it in ruins and useless to both Christians and Muslims.
The Seventh Crusade (1248-1254) under Louis IX of France was motivated by this massacre, but it accomplished little except to play a part in the process of replacement of the Ayyubid sultans with the more powerful Mamluks, who were the Crusaders' main opponents in 1250.