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.
Church of the Holy Sepulchre
Many historians maintain that the main concern of Pope Urban II, when calling for the First Crusade, was the threat to Constantinople from the Turkish invasion of Asia Minor in response to the appeal of Byzantine Emperor Alexios I Komnenos. Historians agree that the fate of Jerusalem and thereby the Church of the Holy Sepulchre was of concern if not the immediate goal of papal policy in 1095. The idea of taking Jerusalem gained more focus as the Crusade was underway. The rebuilt church site was taken from the Fatimids (who had recently taken it from the Abassids) by the knights of the First Crusade on 15 July 1099.
The First Crusade was envisioned as an armed pilgrimage, and no crusader could consider his journey complete unless he had prayed as a pilgrim at the Holy Sepulchre. Crusader Prince Godfrey of Bouillon, who became the first crusader monarch of Jerusalem, decided not to use the title "king" during his lifetime, and declared himself "Advocatus Sancti Sepulchri" ("Protector [or Defender] of the Holy Sepulchre"). By the crusader period, a cistern under the former basilica was rumoured to have been the location where Helena had found the True Cross, and began to be venerated as such; although the cistern later became the "Chapel of the Invention of the Cross," there is no evidence of the rumour before the 11th century, and modern archaeological investigation has now dated the cistern to 11th century repairs by Monomachos.
According to the German clergyman and orient pilgrim Ludolf von Sudheim, the keys of the Chapel of the Holy Sepulchre were in hands of the "ancient Georgians" and the food, alms, candles and oil for lamps were given them by the pilgrims in the south door of the church.