Why was Frederick II’s agreement with al-Kamil unusual in Crusade history?

The 1229 treaty between Frederick II and the Ayyubid sultan al-Malik al-Kamil was unique in the history of the Crusades because it achieved its objectives entirely through diplomacy rather than military confrontation.1

While typical Crusades relied on armed conflict to seize territory, this agreement resulted in the peaceful cession of Jerusalem, Bethlehem, and a corridor to the sea to the kingdom of Jerusalem. The treaty was also notable for its provisions regarding religious coexistence:12
* Muslims retained control of the Temple area, including the Dome of the Rock and the Aqṣā Mosque.1
* Muslim residents were permitted to remain in the city, retaining their property and their own justice system to safeguard their religious interests.1

Despite its diplomatic success, the treaty was highly controversial and denounced by the devout of both faiths, as it lacked the military security typically sought by Crusaders and left the ceded areas difficult to defend.1