While in the 900s Byzantium had been able to reseize territory in Syria, starting around the 1030s the Seldjuk Turks gained power and territory. These incursions prompted the emperor of Byzantium, Alexius I Comnenus to send a delegation to Piacenza, Italy, asking Pope Urban II in March 1095 for help against the Turks. On November 27, 1095, in Clermont, France, Pope Urban II called for a crusade to help the Byzantines and to free the city of Jerusalem. The official start date was set as August 15, 1096. Those armies that left before that time are considered part of the People's Crusade. Three of these armies never made it past Hungary; two others (under Peter the Hermit and Walter Sansavoir) were defeated by Kilij Arslan of Nicaea.
Traditionally, the First Crusade deals with those who left after that official start date. As with the People's Crusade, these crusaders did not form one unified army. Though he required oaths of fealty and the return of any formerly Byzantine lands, Alexius refused leadership of the Crusade and did not take an active role; Stephen of Blois was the leader for a short time until he deserted; Adhémar of Le Puy was able to coordinate the different factions until his death in 1098.
The crusaders first gathered in Constantinople in fall 1096. They besieged Nicaea while Kilij Arslan was away (the city surrendered to Alexius), and later defeated an army commanded by Kilij Arslan at Dorlyaeum. On their progress to Jerusalem the crusaders (or a faction of them) also seized Edessa (which became the first Latin settlement) and Antioch. After Antioch, the crusaders waited out the summer heat and disorder from Adhémar's death. Eventually, the rank and file soldiers forced the crusade to continue. After unsuccessfully besieging `Arqah for three months, the crusaders continued to Jerusalem which they seized on July 15, 1099. The Christian armies defended their conquest in August 1099 by defeating an Egyptian relief army. Pope Urban II died on July 29, 1099, without hearing the news.
Encouraged by the success (and in some cases threatened by excommunication for nonparticipation or desertion), more armies left for Jerusalem in 1100-1101; these armies were defeated in Asia Minor. During subsequent decades fighting (but not crusading) continued in the area. The Byzantines were unhappy about the Franks not returning land that had once belonged to Byzantium; the Franks were unhappy about the lack of support from the Byzantines during the crusade--and the subsequent attempts by the Byzantines to retake territory (as in 1099, 1100, 1104, 1137, 1142, and 1158-9). Bohemund, in fact, organized a campaign (or even a crusade) against Byzantium that was defeated at Durazzo.
In addition, fighting continued between the Franks and the Muslims. While their eastward expansion was stopped at Harran in 1104, and they were defeated at Sarmada in 1119, in general the Franks expanded their territory. They took Acre in 1104, Tripoli in 1109, Beirut and Saida in 1110, and Tyre in 1124, ending by controlling all of the coast to Ascalon. During this time frame, the military orders of the Hospitallers (1113) and Templars (1120) also began. However, around 1130, the crusaders moved from offense to defense, and stayed there until 1153.
On the Muslim side, in 1127, Zangi (or Zengi or Zenghi) became governor of Mosul. He then took control of Aleppo in June 1128 and began adding to his territory from lands Christians (and non-Christians) had controlled. On December 24, 1144, Zangi captured Edessa.
