The first crusade began in 1095 with a request from the Byzantine Emperor Alexios I Komnenon to Pope Urban II for military assistance in keeping the invading Turks from seizing additional Christian lands. In response Urban II called for a crusade, proclaiming that killing of enemies of Christianity was not a sin, and was in fact a holy deed. The response to this decree was enormous, and the crusaders moved towards the Holy Land in three distinct waves, with a purpose not so much to defend Byzantium but instead to recapture the Holy Land of the Levant.
The first wave arrived at Constantinople in 1096. It was a largely disorganized and poorly equipped rabble of peasantry under the leadership of a man known as Peter the Hermit. This force had no obedience to Alexios I, and in fact the Byzantine Emperor quickly came to view them as a threat. This so-called People's Crusade could not be effectively controlled, and indeed on the way to Constantinople Peter's forces repeatedly went against his orders and slaughtered many Jews in central Europe, viewing them as additional enemies of Christ. With the assistance of Alexios I, the army moved into Anatolia where it was destroyed by Turkish forces. A second substantially larger force under the command of French nobility arrived in Constantinople. This force moved through Anatolia and experienced a large degree of success in fighting the Turks, capturing Antioch in 1098 and taking Jerusalem in 1099, massacring many of the city's inhabitants, Muslims, Jews, and Christians alike. The third wave of European forces joined them in the conquered city in 1101.
The final result of the first crusade was the establishment of the Crusader States of the Kingdoms of Jerusalem and Antioch and the Counties of Tripoli and Edessa.