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. Its origin is linked both with the political situation in Catholic Christendom, including the political and social situation in 11th-century Europe, the Gregorian Reform within the papacy,[3] as well as the military's and religious confrontation of Christianity and Islam in the East.
Christianity had been adopted throughout the Roman Empire in Late Antiquity, but in the 7th to 8th centuries, the Umayyad Caliphate had conquered Syria, Egypt, and North Africa from the predominantly Christian Byzantine (Eastern Roman) Empire, and Hispania from the Visigothic Kingdom.[4] In North Africa, the Umayyad empire eventually collapsed and a number of smaller Muslim kingdoms emerged, such as the Aghlabids, who attacked Italy in the 9th century. Pisa, Genoa, and the Principality of Catalonia began to battle various Muslim kingdoms for control of the Mediterranean Basin, exemplified by the Mahdia campaign of 1087 and battles at Majorca and Sardinia.[5]
Between the years of 1096 and 1101, the Byzantine Greeks experienced the crusade as it arrived in Constantinople in three separate waves. In the early summer of 1096, the first large unruly group arrived on the outskirts of Constantinople. This wave was reported to be undisciplined and ill-equipped as an army. This first group is often called the Peasants' Crusade or the People's Crusade. 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. Together, this group and the first wave numbered an estimated 60,000.[6][7]
The second wave was led by Hugh I, Count of Vermandois, the brother of King Philip I of France. Also among the second wave were Raymond IV, Count of Toulouse and the army of Provençals. "It was this second wave of crusaders which later passed through Asia Minor, captured Antioch in 1098 and finally took Jerusalem 15 July 1099."[8]
The third wave, composed of contingents from Lombardy, France, and Bavaria, arrived in Jerusalem in the early summer of 1101.[9]
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