In 1244 the city of Jerusalem was seized by Muslim forces. The Pope, Innocent IV, called for a new crusade, but obtaiend little popular support. He was, in any event, himself embroiled in a struggle in Europe, wirth the Holy Roman Emperor Frederick II, which made it impossible for the Empire to support him, and the English, under Henry III were also engaged in a local conflict, between the king and his nobility, led by Simon de Montfort. This left France as the strongest Crusader state in Europe, and France was wary of both England and the Empire who might, were a crusade to be undertaken, take the opportunity to encroach on French possessions.
However, after a treaty was signed with England, Louis declared, in 1245, his intention to go to the East. He was also able to enlist to his side two of his brothers, Alphonse of Poitiers and Charles of Anjou. Preparations took 3 years and in 1248 an army of about 15000 set sail from southern France.
They sailed to Cyprus, to winter there, and to negotiate with other local powers (the Latin Empire in Byzantium, the Principality of Antioch, and the Knights Templar, each of whom wanted Louis' help in military endeavours of their own.
Louis' target, however, was Egypt, where he hoped to establish a base, with sea routes, and plentiful grain, from which then to launch his campagn to retake Jerusalem. He landed in Damietta, but the Egyptians simply withdrew and waited for the Nile floods to bog the Crusade army down. It was not until the autumn that he was able to advance on Cairo, but when Robert of Artois was killed in a side campaign against Gidelia and Mansourah, the Crusaders beseigned Mansourah, lost the initiative, and ran out of food.
The following year (1250) worse was to come: during a retreat to Damietta, Louis was attacked and captured, with his army being heavily defeated. He fell ill with dysentery, was cured by an Arab physician, and swore to surrender Damietta, and to pay 50,000 gold bezants for his ransom. His wife, Marguerite, raised half and Louis, once free, defaulted on the rest, and went to Acre, which was still a Christian enclave.
There he attempted to negotiate treaties with a variety of posible allies, including the Syrian Muslims (thought to be rivals to the Egyptians, and the Khan of the Mongols (who suggested that Louis submit himself to the Khan's rule, rather than continue his campaign).
In 1254 Louis' money ran out and, while he left a garrsion at Acre, he withdrew to France having achieved little. It did, however, give him prestige at home, and in Europe, for having made the Crusade, and in 1270 he was to attempt a second, which was also to end in failure.