After capturing Jerusalem on July 15, 1099, most of the European crusaders felt their mission was completed. Many of them began to make preparations to leave for their homes. On July 22, Godfrey of Bouillon was appointed "Guardian of the Holy Sepulchre." Shortly afterwards, ambassadors from Fatimid-ruled Egypt arrived, demanding that the "Franks" leave Jerusalem immediately, as the Fatimids were the previous rulers of the Holy City. The Muslims were totally ignored. However, in early August, scouting reports indicated that a large Fatimid army was closing on Jerusalem.

On August 10, Godfrey gathered his forces and marched westward to the port of Ascalon, where the Fatimid army was reported to be encamped. Robert Count of Flanders and his troops also accompanied Godfrey, as did Arnulf of Chocques – a cleric recently appointed Patriarch of Jerusalem – who carried a relic of the True Cross, discovered in the Holy City on August 5. On August 11, the crusaders approached their target. Godfrey's scouts reported that the Fatimid army was camped just outside of the port. Apparently, the enemy had no inkling that the "Franks" had struck out from Jerusalem, and were equally unaware that the crusaders were so close.

Early in the morning of August 12, the crusaders' scouts reported that the Fatimid camp was quiet, with no idea that their enemy was bearing down on them. As the "Franks" approached the Egyptian camp just after dawn, the Fatimid pickets saw a tremendous cloud of dust approaching. This tactic supposed to frighten the Egyptians and this task was accomplished perfectly. Contemporary histories say this battle was over very quickly - the suddenness and surprise of the crusader attack was too much for the Egyptians. By mid-morning, the Egyptian army had had enough; the survivors left the battlefield and headed pell-mell for the port of Ascalon. The crusaders spent the night in the abandoned Egyptian camp, taking as much plunder as they could, then burnt the rest. The army returned to the Holy City the next day.