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.
Seljuk Empire
During the First Crusade, the fractured states of the Seljuqs were generally more concerned with consolidating their own territories and gaining control of their neighbours than with cooperating against the crusaders. The Seljuqs easily defeated the People's Crusade arriving in 1096, but they could not stop the progress of the army of the subsequent Princes' Crusade, which took important cities such as Nicaea (İznik), Iconium (Konya), Caesarea Mazaca (Kayseri), and Antioch (Antakya) on its march to Jerusalem (Al-Quds). In 1099 the crusaders finally captured the Holy Land and set up the first Crusader states. The Seljuqs had already lost Palestine to the Fatimids, who had recaptured it just before its capture by the crusaders.