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Four years in the Kingdom of Jerusalem

Upon his liberation from captivity in Egypt, Louis IX devoted four years to fortifying the Kingdom of Jerusalem, focusing his efforts in Acre, Caesarea, and Jaffa. He generously utilized his resources to aid the Crusaders in reconstructing their defenses and actively engaged in diplomatic endeavors with the Ayyubid dynasty. In the spring of 1254, Louis and his remaining forces made their return to France.

Louis maintained regular correspondence and envoy exchanges with the Mongol rulers of his era. During his first crusade in 1248, he received envoys from Eljigidei, the Mongol military leader stationed in Armenia and Persia. Eljigidei proposed that Louis should launch an offensive in Egypt while he targeted Baghdad to prevent the unification of the Muslim forces in Egypt and Syria. In response, Louis sent André de Longjumeau, a Dominican priest, as a delegate to the Khangan Güyük Khan (r. 1246–1248) in Mongolia. However, Güyük's death preceded the arrival of the emissary, and his widow and acting regent, Oghul Qaimish, rejected the diplomatic proposition.

Louis sent another representative, the Franciscan missionary and explorer William of Rubruck, to the Mongol court. Rubruck visited the Khagan Möngke (r. 1251–1259) in Mongolia and spent several years there. In 1259, Berke, the leader of the Golden Horde, demanded Louis's submission. In contrast, Mongol emperors Möngke and Khubilai 's brother, the Ilkhan Hulegu, sent a letter to the French king, soliciting his military aid; this letter, however, never reached France.