Louis IX was no fool. He was, by most accounts, the most pious and capable monarch in Christendom, later canonized as a saint by the Catholic Church. His plan for the Seventh Crusade followed the same logic that had drawn the Fifth Crusade to Egypt a generation before: take Damietta, use its wealth and grain to sustain an army, then push toward Jerusalem. Egypt's agricultural riches would keep the Crusaders fed where the barren Levantine coast could not. What Louis did not account for -- or chose to ignore -- was the Nile. The river that made Egypt wealthy also made it unconquerable. When the annual flood came, it swallowed the delta, turning roads into canals and fields into lakes. Louis and his army were stranded.

The flood pinned the Crusaders at Damietta from June through November 1249. What should have been a staging ground for an advance on Cairo became a stagnant camp where knights, far from home and restless, "sat back and enjoyed the spoils of war," as the chronicles record. Discipline frayed. Louis established an archbishop in the city -- Gilles of Saumur, placed under the authority of the Latin Patriarch of Jerusalem, Robert of Nantes -- and used Damietta as a base for directing operations against Muslim positions in Syria. But these were administrative gestures, not military momentum. Meanwhile, Louis ignored the agreement from the Fifth Crusade that Damietta should be handed to the Kingdom of Jerusalem, by then a diminished rump state clinging to the coastal city of Acre. He kept the prize for France.

When the floodwaters finally receded and Louis marched his army south toward Cairo, the campaign unraveled. The Egyptian forces, regrouped and reinforced, harried the Crusaders along the Nile. Supply lines stretched and broke. Disease tore through the ranks. The Battle of Al Mansurah in February 1250 was a catastrophe -- Louis's brother, Robert of Artois, was killed in a reckless charge into the city, and the main Crusader force was mauled. By April 1250, Louis himself was captured along with much of his army. The ransom for the king was staggering: 400,000 livres and the return of Damietta. The city that fell so easily was handed back just as easily, the entire Seventh Crusade's Egyptian campaign reduced to a costly round trip.

Standing at the mouth of the Nile, Damietta beckoned every generation of Crusaders with the same seductive promise: control this port and you control the wealth of Egypt, and with Egypt's wealth, you can buy back Jerusalem. The Fifth Crusade tried in 1218 and failed. Louis IX tried in 1249 and failed. The city's position at the delta made it easy to approach by sea but impossible to hold once the Nile asserted itself. From the air today, the flat delta stretches green and intricate where the great river splits into its final channels before the Mediterranean. The fortifications are long gone, but the geography that defeated two Crusades remains exactly as it was -- the patient, annual flood cycle that no army from Europe ever learned to respect.