In 1244, shortly after the expiry of the ten-year truce of the Sixth Crusade, the Khwarezmians retook Jerusalem. The fall of Jerusalem was 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. This time, there was no popular enthusiasm for a new crusade.
Pope Innocent IV and Frederick II, Holy Roman Emperor continued the papal-imperial struggle. Frederick had captured and imprisoned clerics on their way to the Council of Lyons, and in 1245 he was formally deposed by Innocent IV. Pope Gregory IX had also earlier offered King Louis' brother, count Robert of Artois, the German throne, but Louis had refused. Thus, the Holy Roman Emperor was in no position to crusade. Henry III of England was still struggling with Simon de Montfort and other problems in England. Henry and Louis were not on the best of terms, being engaged in the Capetian-Plantagenet struggle, and while Louis was away on crusade the English king signed a truce promising not to attack French lands. Louis IX had also invited King Haakon IV of Norway to crusade, sending the English chronicler Matthew Paris as an ambassador, but again was unsuccessful. The only man interested in beginning another crusade therefore was Louis IX, who declared his intent to go East in 1245.
France was perhaps the strongest state in Europe at the time, as the Albigensian Crusade had brought Provence into Parisian control. Poitou was ruled by Louis IX's brother Alphonse of Poitiers, who joined him on his crusade in 1245. Another brother, Charles I of Anjou, also joined Louis. For the next three years Louis collected an ecclesiastical tenth (mostly from church tithes), and in 1248 he and his approximately 15-25,000-strong army that included 1,500 knights, and 5,000 crossbowmen sailed from the ports of Aigues-Mortes, which had been specifically built to prepare for the crusade, and Marseille. Louis IX's financial preparations for this expedition were comparatively well organised, and he was able to raise approximately 1,500,000 livres tournois. However, many nobles who joined Louis on the expedition had to borrow money from the royal treasury, and the crusade turned out to be very expensive.
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.
Nonetheless, Egypt was the object of his crusade, and he landed in 1249 at Damietta on the Nile. Egypt 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. The flooding of the Nile had not been taken into account, however, and it soon grounded Louis and his army at Damietta for six months, where the knights sat back and enjoyed the spoils of war. Louis ignored the agreement made during the Fifth Crusade that Damietta should be given to the Kingdom of Jerusalem, now a rump state in Acre, 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 Ayyub, died. A force led by Robert of Artois and the Templars attacked the Egyptian camp at the Battle of 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 showing utter agony, a Templar knight lamented :Template:Cquote
In March of 1250 Louis finally returned to Damietta, but he was taken captive on the way there, fell ill with dysentery, and was cured by an Arab physician. In May he was ransomed in return for Damietta and 400,000 livres, and he immediately left Egypt for Acre, one of few remaining crusader possessions in Syria. In 1250, Turanshah, as-Salih's successor, was killed by the Mamluks whom they thought was making a distinction against them.
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 begun to appear in the east and who the Christians, encouraged by legends of a Nestorian kingdom among them (cf. Prester John), hoped would help them fight the Muslims and restore the Crusader States. They, like the Muslims who were similarly 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. Louis' embassy to the Mongol ruler Möngke Khan, headed by William of Rubruck, was a failure. The Khan 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.
- Memoirs of Jean de Joinville, from the University of Virginia