Military history of the Crusader states
The Seljuk–Crusader War began when the First Crusade wrested territory from the Seljuk Turks during the Siege of Nicaea in 1097 and lasted until 1128 when Zengi became atabeg of Aleppo. At the latter date, the chief threat to the Crusaders from the east and north became the Zengids. The conflict was generally fought between European Crusaders and the Seljuk Turks and their vassals. The Muslim Syrian emirates occasionally allied themselves with the Christians against rival states.
Crusades
Five major sources of information exist on the Council of Clermont that led to the First Crusade: the anonymous " Gesta Francorum " (The Deeds of the Franks), dated about 1100–01; Fulcher of Chartres, who attended the council; Robert the Monk, who may have been present, and the absent and Guibert de Nogent. These retrospective accounts differ greatly. In his 1106–07 "Historia Iherosolimitana", Robert the Monk wrote that Urban asked western Roman Catholic Christians to aid the Orthodox Byzantine Empire because "Deus vult" ("God wills it") and promised absolution to participants; according to other sources, the pope promised an indulgence. In these accounts, Urban emphasises reconquering the Holy Land more than aiding the emperor, and lists gruesome offences allegedly committed by Muslims. Urban wrote to those "waiting in Flanders" that the Turks, in addition to ravaging the "churches of God in the eastern regions", seized "the Holy City of Christ, embellished by his passion and resurrection – and blasphemy to say it – have sold her and her churches into abominable slavery". Although the pope did not explicitly call for the reconquest of Jerusalem, he called for military "liberation" of the Eastern Churches.