The First Crusade was a military expedition by European Christians to regain the Holy Lands taken by the Muslim conquest of the Levant, which resulted in the capture of Jerusalem in 1099. It was launched in 1095 by Pope Urban II with the primary goal of responding to the appeal from Byzantine Emperor Alexius I. The Emperor requested that western volunteers come to their aid and repel the invading Seljuk Turks from Anatolia, modern day Turkey. 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, both knights and peasants from many nations of Western Europe traveled over land and by sea first towards Constantinople (modern-day Istanbul) and then towards Jerusalem as crusaders, with the numbers of the peasants greatly outnumbering the numbers of the knights. Because the peasants and knights were split in different armies, only the knights' army reached Jerusalem. Once there, the crusaders set up a siege and captured the city in July 1099, establishing the Kingdom of Jerusalem, County of Tripoli, the Principality of Antioch, and the County of Edessa.
Given that the First Crusade was largely concerned with Jerusalem, a city which had not been under Christian dominion for 461 years, as well as the crusader army's refusal to return the land to Byzantine control, the status of the First Crusade as defensive or as aggressive in nature remains controversial, both within academias and without.
Although these gains lasted for less than two hundred years, the First Crusade was part of the Christian response to the Islamic conquests, as well as the first major step towards reopening international trade in the West since the fall of the Western Roman Empire.