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.

Blessed Pope Urban II (ca. 1035 – 29 July 1099), born Otho de Lagery (alternatively: Otto, Odo or Eudes), was Pope from 12 March 1088 until his death on 29 July 1099. He is best known for starting the First Crusade (1096–1099) and setting up the modern-day Roman Curia in the manner of a royal court to help run the Church.

Pope Gregory VII named him cardinal-bishop of Ostia ca. 1080. He was one of the most prominent and active supporters of the Gregorian reforms, especially as legate in Germany in 1084 and was among the few whom Gregory VII nominated as possible successors to be Pope. Desiderius, abbot of Monte Cassino was first chosen Pope as Victor III when Gregory VII died in 1085, but after Victor's short reign, Otho was elected Pope Urban II by acclamation (March 1088) at a small meeting of cardinals and other prelates held in Terracina in March 1088. He took up the policies of Pope Gregory VII, and while pursuing them with determination, showed greater flexibility and diplomatic finesse. At the outset, he had to reckon with the presence of the powerful antipope Clement III in Rome, but a series of well-attended synods held in Rome, Amalfi, Benevento, and Troia supported him in renewed declarations against simony, Investiture Controversy, clerical marriages, and continued opposition to Emperor Henry IV.