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
This situation of multiple possible causes or motivations for the crusades has led to extended debates in modern historiography. A theory proposed by Carl Erdmann (1935), known as the "Erdmann thesis", links the First Crusade l to the Gregorian Reform, the series of reforms initiated by Pope Gregory VII during c. 1050–80. These reforms re-established the Western Church, which had been much weakened during the 10th century, as a relevant power able to inspire and co-ordinate the crusades. 
The "Erdmann thesis" proposes that the strengthened Western Church, combined with the weakened Byzantine Empire, permitted a projection of power from west to east, with the actual conquest of Jerusalem figuring at best as a secondary, popular goal.