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.
First Crusade
The great French nobles and their trained armies of knights, however, were not the first to undertake the journey towards Jerusalem. Urban had planned the departure of the first crusade for 15 August 1096, the Feast of the Assumption, but months before this, a number of unexpected armies of peasants and petty nobles set off for Jerusalem on their own, led by a charismatic priest called Peter the Hermit. Peter was the most successful of the preachers of Urban's message, and developed an almost hysterical enthusiasm among his followers, although he was probably not an "official" preacher sanctioned by Urban at Clermont.