Fifth Crusade
Trapped
The Christians were in a death trap. When they marched up into the angle formed by the Nile and the Bahr as-Saghir, the army had crossed a dry canal. No one thought to consider it a danger. Worse, Pelagius had neglected to bring adequate supplies, thinking to capture the enemy supplies quickly. A Christian fleet of about six hundred ships was in the Nile—enough to keep the army re-supplied at to support its advance. Once it was clear that Mansourah could not be taken quickly, fortifications were built on the other two sides of the triangle.
There were plenty of locals to tell the Crusaders how easily the Muslims could trap them, but Pelagius would not listen. King John again urged acceptance of al-Kamil's terms, but rejecting his advice had become almost automatic by this time. A couple of weeks passed in this manner.
It was August, and the Nile was rising a little every day. Soon, the waters were high enough and water flowed into the canal. Within a few days, the levels were high enough to send ships down it, cutting the Christians off from retreat and from being re-supplied from Damietta. The fleet was trapped, the army was trapped, and there was food enough for only twenty days.
Many in the army urged immediate retreat, before their situation grew worse. At last awakening to the peril, Pelagius agreed. On the night of August 26, without ever having fought a battle, the great army began its retreat.