The First Crusade (1096–1099) was a military expedition by Roman Catholic Europe to regain the Holy Lands taken in the Muslim conquests of the Levant (632–661), ultimately resulting in the recapture of Jerusalem in 1099. It was launched on 27 November 1095 by Pope Urban II with the primary goal of responding to an appeal from Byzantine Emperor Alexios I Komnenos, who requested that western volunteers come to his aid and help to repel the invading Seljuq Turks from Anatolia.
An additional goal soon became the principal objective—the Christian reconquest of the sacred city of Jerusalem and the Holy Land and the freeing of the Eastern Christians from Islamic rule. During the crusade, knights and peasants from many nations of Western Europe travelled over land and by sea, first to Constantinople and then on towards Jerusalem, as crusaders; the peasants greatly outnumbered the knights.
Peasants and knights were split into separate armies; however, because the peasants were not as well-trained in combat as the knights, their army failed to reach Jerusalem. The knights arrived at Jerusalem, launched an assault on the city, and captured it in July 1099, massacring many of the city’s Muslim and Jewish inhabitants. They also established the crusader states of the Kingdom of Jerusalem, the County of Tripoli, the Principality of Antioch, and the County of Edessa.
Because the First Crusade was largely concerned with Jerusalem, a city which had not been under Christian dominion for 461 years, and the crusader army refused to return the land to the control of the Byzantine Empire, the status of the First Crusade as defensive or as aggressive in nature remains controversial. The First Crusade was part of the Christian response to the Muslim conquests, and was followed by the Second Crusade to the Ninth Crusade, but the gains made lasted for less than 200 years. It was also the first major step towards reopening international trade in the West since the fall of the Western Roman Empire.