In 1095 at the Council of Piacenza, Byzantine Emperor Alexios I Komnenos requested support from Pope Urban II against the Seljuk threat. What the Emperor probably had in mind was a relatively modest force, and Urban far exceeded his expectations by calling for the First Crusade at the later Council of Clermont. Within a year, tens of thousands of people, both commoners and aristocrats, departed for the military campaign. Individual crusaders' motivations to join the crusade varied, but some of them probably left Europe to make a new permanent home in the Levant.

Alexios cautiously welcomed the feudal armies commanded by western nobles. By dazzling them with wealth and charming them with flattery, Alexios extracted oaths of fealty from most of the Crusader commanders. As his vassals, Godfrey of Bouillon, nominally duke of Lower Lorraine, the Italo-Norman Bohemond of Taranto, Bohemond's nephew Tancred of Hauteville, and Godfrey's brother Baldwin of Bologne all swore that any territory gained which the Roman Empire had previously held, would be handed to Alexios' Byzantine representatives. Only Raymond IV, Count of Toulouse refused this oath, instead promising non-aggression towards Alexios.

The crusaders marched along the Mediterranean coast to Jerusalem. On 15 July 1099, crusaders took the city after a siege lasting barely longer than a month. Thousands of Muslims and Jews were killed, and the survivors sold into slavery. Proposals to govern the city as an ecclesiastical state were rejected. Raymond refused the royal title, claiming only Christ could wear a crown in Jerusalem. This may have been to dissuade the more popular Godfrey from assuming the throne, but Godfrey adopted the title Advocatus Sancti Sepulchri ('Defender of the Holy Sepulchre') when he was proclaimed the first Frankish ruler of Jerusalem.

The foundation of these three crusader states did not change the political situation in the Levant profoundly. Frankish rulers replaced local warlords in the cities, but large-scale colonisation did not follow, and the new conquerors did not change the traditional organisation of settlements and property in the countryside. The Frankish knights regarded the Turkic mounted warlords as their peers with familiar moral values, and this familiarity facilitated their negotiations with the Muslim leaders. The conquest of a city was often accompanied by a treaty with the neighbouring Muslim rulers who were customarily forced to pay a tribute for the peace. The crusader states had a special position in Western Christianity's consciousness: many Catholic aristocrats were ready to fight for the Holy Land, although in the decades following the destruction of the large Crusade of 1101 in Anatolia, only smaller groups of armed pilgrims departed for Outremer.