The ideas that drove the First Crusade were born out of a general sense in Christian Europe that, because the first millennium was approaching, the return of Christ was surely nigh. As such, it was important that the Holy Lands should be returned to Christian hands. In 1095, Pope Urban II gave his famous "Deus volt" speech, a call to arms that echoed across Europe and encouraged many to set out for the Middle East to kill the Saracens and recapture Jerusalem. He was partly motivated to do this by a request from Alexius Comnenus, the Byzantine Emperor, for help in fighting the Turks.
There were many reasons for people to join the Crusade. While many were religiously motivated, the Crusades were also seen as a means of acquiring new land and personal wealth. It is estimated that up to 100,000 people journeyed towards Jerusalem in this first wave of Crusaders. Key figures in the First Crusade include Peter the Hermit, a religious man who acquired a significant following of less organized Crusaders (The People's Crusade), and the leaders of the four key organized armies: Raymond of St Gilles, Godfrey of Bouillon, Bohemond of Taranto, and Hugh of Vermandois.
The People's Crusade was poorly organized and inexperienced for the most part, and as it did not wait for the main body of Crusaders, it suffered an early defeat after crossing the Bosphorous. The other four armies, however, allied with Alexius Comnenus, the Byzantine Emperor upon their arrival in Constantinople. This combined force conquered Nicea and subsequently Antioch, following a prolonged siege. Eventually, the Crusaders proceeded on to Jerusalem. The Holy City was conquered after a further siege on 13 July, 1099.
The First Crusade was only the first in a series of religious wars in the Middle East which would continue for centuries, with the city of Jerusalem changing hands several times. Many Jews, as well as Muslims, were killed by Crusaders on their journey across Europe and the Middle East, and subsequent Crusades would also sweep up Christian "heretics" in their path.