In 1244 the Khwarezmians retook Jerusalem, after the end of a 10-year truce following the Sixth Crusade. The fall of Jerusalem, no longer an earth-shattering event to European Christians who had seen the city pass from Christian to Muslim control numerous times in the past two centuries, did not prompt an immediate call for a new crusade. Pope Innocent IV and Frederick II, Holy Roman Emperor were more concerned with the struggle between the Guelphs and Ghibellines, and Henry III of England was still struggling with Simon de Montfort and other problems in England. The only man interested in beginning another crusade was Louis IX, who declared his intent to go the East in 1245.
France was perhaps the strongest state in Europe at the time, as Provence had mostly fallen under Parisian control after the Albigensian Crusade, and Toulouse was led Louis IX's brother Alphonse, who joined him on his crusade in 1245. Another brother, Charles I of Anjou, also joined Louis. For the next three years Louis collected money (mostly from church tithes), and in 1248 he and his approximately 20 000-strong army sailed from the ports of Aigues-Mortes and Marseilles.
They sailed first to Cyprus and spent the winter on the island, negotiating with various other powers in the east; the Latin Empire set up after the Fourth Crusade asked for his help against the Byzantine Empire of Nicaea, and the Principality of Antioch and the Knights Templar wanted his help in Syria, where the Muslims had recently captured Sidon. However, Egypt was the object of his crusade, and he landed in 1249 at Damietta on the Nile. Egypt was the object of the crusade, as it would, Louis thought, provide a base from which to attack Jerusalem, and its wealth and supply of grain would keep the crusaders fed and equipped.
On June 6 Damietta was taken with little resistance from the Egyptians, who withdrew further up the Nile. Louis ignored the agreement made during the Fifth Crusade that Damietta should be given to the nominal King of Jerusalem (at the time John of Brienne; in 1249 Conrad IV of Germany), but he did set up an archbishopric there (under the authority of the Latin Patriarch of Jerusalem) and used the city as a base to direct military operations against the Muslims of Syria.
In November, Louis marched towards Cairo, and almost at the same time, the Ayyubid sultan of Egypt, as-Salih, died. A force led by Robert of Artois and the Templars attacked the Egyptian camp at al-Mansourah, but they were defeated and Robert was killed. Meanwhile, Louis' main force was attacked by the Mameluk Baibars, the commander of the army and a future sultan himself. Louis was defeated as well, but he did not withdraw to Damietta for months, preferring to besiege Mansourah, which ended in starvation and death for the crusaders rather than the Muslims. In March of 1250 Louis finally returned to Damietta, but he was taken captive on the way there. In May he was ransomed in return for Damietta and 400 000 livres, and he immediately left Egypt for Acre, one of the few remaining possessions of the Christians in Syria. Meanwhile, the Mameluk soldiers of Egypt revolted. Turanshah, as-Salih's successor, and took control of Cairo, creating a Mameluk dynasty that would eventually conquer the last of the crusader territories.
Louis made an alliance with the Mameluks, and from his new base in Acre began to rebuild the other crusader cities. Although the Kingdom of Cyprus claimed authority there, Louis was the de facto ruler. Louis also negotiated with the Mongols, who had began to appear in the east and whom the Christians hoped would help them fight the Muslims and restore the Crusader States. They, like the Muslims who were similarily negotiating with the Mongols against the Christians, were unaware that the Mongols were not interested in helping either side and would eventually be disastrous for both. The Mongol Khan Mongke rejected Louis' invitation to convert to Christianity, and instead suggested Louis submit to him.
In 1254 Louis' money ran out, and his presence was needed in France where his mother and regent Blanche of Castile had recently died. His crusade was a failure, but he was considered a saint by many, and his fame gave him an even greater authority in Europe than the Holy Roman Emperor. In 1270 he attempted another crusade, though it too would end in failure.
The history of the Seventh Crusade was written by Jean de Joinville, who was also a participant.