Philip I, Count of Flanders joined Raymond of Tripoli's expedition to attack the Saracen stronghold of Hama in northern Syria. A large crusader army, the Knights Hospitaller and many Templar knights followed him. This left the Kingdom of Jerusalem with very few troops to defend its various territories. Meanwhile, Saladin was planning his own invasion of the Kingdom of Jerusalem from Egypt. When he was informed of the expedition north, he wasted no time in organising a raid and invaded the kingdom with an army of some 30,000 men. Learning of Saladin's plans, Baldwin IV left Jerusalem with, according to William of Tyre, only 375 knights to attempt a defense at Ascalon.

Saladin continued his march towards Jerusalem, thinking that Baldwin would not dare to follow him with so few men. He attacked Ramla, Lydda and Arsuf, but because Baldwin was supposedly not a danger, he allowed his army to be spread out over a large area, pillaging and foraging. However, unknown to Saladin, the forces he had left to subdue the King had been insufficient and now both Baldwin and the Templars were marching to intercept him before he reached Jerusalem. The Christians, led by the King, pursued the Muslims along the coast, finally catching their enemies at Mons Gisardi, near Ramla.

The 16-year-old Baldwin IV of Jerusalem, seriously afflicted by leprosy, led an outnumbered Christian force against Saladin's troops in what became one of the most notable engagements of the Crusades. The Muslim army was quickly routed and pursued for twelve miles. Saladin fled back to Cairo, reaching the city on 8 December, with only a tenth of his army.