If the Arabic sources are to be believed, Frederick II maintained a correspondence with Fakhr al-Din after his return to Europe. They record a letter purportedly from Frederick dated 23 August 1229 at
Barletta, in which Frederick says, "As we explained to you in Sidon, the pope has treacherously and deceitfully taken one of our fortresses, called
Montecassino", referring to the
papal invasion of Sicily. Fakhr al-Din was present at the
siege of Damascus between June and December 1229, acting as al-Kamil's envoy to the besieged
an-Nasir Da'ud. When al-Kamil died on 6 March 1238, he was succeeded by his sons, in Egypt by
al-Adil II and in Syria, by
al-Salih Ayyub. In 1239 the treaty with Frederick expired, and Jerusalem again was threatened. The next year al-Salih Ayyub defeated his brother and became sultan of Egypt. The
Barons' Crusade of 1239–1241 increased the size of the
Kingdom of Jerusalem, successfully pitting al-Salih Ayyub against his cousin
al-Salih Ismail, now
emir of Damascus. At al-Salih Ayyub's invitation, the
Khwarezmians advanced through Syria and Palestine and on 15 June 1244 successfully conducted the
Siege of Jerusalem, leaving the city in ruins. In October 1245, al-Salih Ayyub, again allied to the Khwarezmians, defeated al-Salih Ismail, allied with the kingdom, at the
Battle of La Forbie. In 1246, deciding that his Khwarezmian allies were dangerously uncontrollable, he turned on them and defeated them near Homs, killing their leader and dispersing the remnants throughout Syria and Palestine. In 1247, Fakhr al-Din was a commander of the troops of sultan al-Salih Ayyub during his campaign through Palestine. He captured
Tiberias, and at
Ascalon, he took the castle rebuilt by
Theobald I of Navarre and began dismantling it on 24 October 1247. This was the first Muslim offensive against the Crusaders since 1189. == The Seventh Crusade ==