Why did thousands of Europeans march to Jerusalem in 1096?

They answered a call that began as a request for help. In 1095 the Byzantine emperor Alexios I sent envoys asking for military support against the Seljuk-led Turks, and at the Council of Clermont Pope Urban II gave a speech supporting the request and urging an armed pilgrimage to Jerusalem. Many who went were moved by personal piety, though historians say it is hard to assess the motives of thousands of people whose stories were told mostly by monks and clerics.

Clermont was not just about the crusade. It was primarily a synod about Church reforms, decrees and local issues, and only the speech on its last day made it famous. The five recorded versions of the speech differ widely, but all but one agree that Urban talked about violence in European society, helping the Greeks, crimes against Christians in the east, and an armed pilgrimage with remission of sins for any who died. They do not all mention Jerusalem as the goal.

The response was huge, and it was not controlled. Thousands of poor Christians led by Peter the Hermit set off first, in what became known as the People's Crusade, and anti-Jewish violence, the Rhineland massacres, broke out in France and Germany. The People's Crusade was annihilated in a Turkish ambush in October 1096.

In July 1099 the crusader armies stormed Jerusalem, and a large fraction of the residents were massacred. Crusader eyewitnesses leave little doubt about the great slaughter, though some historians think later medieval sources exaggerated its scale. Most crusaders then went home, and only 300 knights and 2,000 infantry remained to defend Palestine.