The causes of the Crusades in general, and particularly of the First Crusade, is widely debated among historians.While the relative weight or importance of the various factors may be the subject of ongoing disputes, it is clear that the First Crusade came about from a combination of factors in both Europe and the Near East.During their conquests, the crusaders established the Latin Rite crusader states of the Kingdom of Jerusalem, the County of Tripoli, the Principality of Antioch, and the County of Edessa.This was contrary to the wishes of the Eastern Rite Byzantines, who wanted the land that the Muslims took from them returned, rather than occupied by Latin Catholics.
At the same time, the reform-minded papacy came into conflict with the Holy Roman Emperors, resulting in the Investiture Controversy.
It was intermittently ideological, as evidenced by the Codex Vigilanus compiled in 881.
Shortly before the First Crusade, Pope Urban II had encouraged the Iberian Christians to reconquer Tarragona, using much of the same symbolism and rhetoric that was later used to preach the crusade to the people of Europe.
It was led by Peter the Hermit and Walter Sans Avoir.
The second wave was also not under the command of the Emperor and was made up of a number of armies with their own commanders.