The town of Mansurah (mod. El-Mansûra, Egypt) was founded by the Ayyūbid sultan al-Kāmil (1218-1238) as a forward military base against the Fifth Crusade (1217-1221), which in November 1219 had seized the vital port of Damietta at the mouth of the eastern branch of the Nile following a prolonged siege.
Mansurah was, in fact, a large fortified encampment of a type typical in Middle Eastern Islamic warfare. Its location also dominated the eastern Nile and the Bahr al-Saghir, a strategic waterway linking the Nile and Lake Manzala. After a long pause, largely caused by the divided leadership of King John of Jerusalem and Cardinal Pelagius, the crusader army advanced along the eastern bank of the Nile in July and August 1221, heading for Cairo. It was, however, halted by the Ayyūbid forces at Mansurah, and al-Kāmil ordered that the irrigation dykes be broken, and the surrounding land flooded. The crusader army found itself caught on a small island between the eastern Nile and the Bahr al-Saghir and was obliged to negotiate a humiliating peace. However, in return for the surrender of Damietta, still held by a crusader garrison, the trapped army was permitted to retreat in safety at the end of August 1221.
In 1249 Damietta again fell to a crusade army, led by King Louis IX of France. Although he was dying, the sultan al- Sālih (1240-1249) assembled an army at Mansurah, supported by a river fleet. In November-December 1249, the crusaders advanced up the Nile toward Mansurah. The death of al-Sālih on 23 November was kept a secret from his army, which skirmished with the crusaders outside the town during December and January. Eventually the crusaders crossed the Bahr al-Saghir to attack the town, but on 11 February 1250 the king’s brother Robert, count of Artois, disobeyed orders and entered Mansurah, where he was defeated in street fighting. The Egyptians then counterattacked, and the crusaders were besieged in their camp, while the Egyptian river fleet won control of the Nile. In March and April the crusaders retreated toward Damietta before being forced to surrender near Fariskur, where King Louis was taken prisoner. In May 1250 some senior crusader leaders were released after paying large ransoms, but much of their army was enslaved.
This second battle of Mansurah was one of the most important during the entire crusades, confirming three strategic points: that Egypt was the center of Islamic power in the Middle East, that Frankish power in the Holy Land could only be preserved by dominating Egypt, and that the conquest of Egypt by a seaborne assault was probably impossible, given the military technology of this period. The Ayyûbid sultanate collapsed during this campaign, to be replaced by a military regime, which evolved into the Mamlûk sultanate. Victory at Mansura gave the Mamlûks great prestige, helping them to inflict a major defeat upon the invading Mongols a decade later.