The trigger for the First Crusade was Emperor Alexius I's appeal to Pope Urban II for mercenaries to help him resist Muslim advances into territory of the Byzantine Empire. However, the Pope called for a large invasion force to not merely defend the Byzantine Empire but also retake Jerusalem.
Proceeding down the Mediterranean coast, the crusaders encountered little resistance, as local rulers preferred to make peace with them and furnish them with supplies rather than fight. On 7th June 1099, the Crusaders reached Jerusalem, with just 12,000 men including 1,500 cavalry remaining, by some estimates. With insufficient troops, supplies, and time, they had no hope of blockading the city, and so resolved to take the city by assault.
In their first assault, they were able to scale the outer wall but were repulsed from the inner one. Their morale was raised when a priest, Peter Desiderius, claimed to have had a divine vision instructing them to fast and then march in a barefoot procession around the city walls, after which the city would fall, following the biblical story of Joshua's siege of Jericho.
The final assault began on 13th July, as one group attacked the south gate and other contingents attacked the northern wall. Initially the troops at the southern gate made little headway, but the contingents at the northern wall fared better, with a slow but steady attrition of the defence. On 15th July, a final push was launched at both ends of the city, and eventually the inner rampart of the northern wall was captured. In the ensuing panic, the defenders abandoned the walls of the city at both ends, allowing the Crusaders to enter.
The Crusaders set about a massacre of the entire population, whether Christian, Jew or Muslim, with one report saying that God would decide their fate in heaven. However, some historians believe that the scale of the massacre has probably been exaggerated.
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They slaughtered the Jewish and Muslim defenders of the city, except for a few who they held for ransom. Accounts of the slaughter say that some of the streets were knee deep in blood.