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.
Christianity in the 11th century
The Crusades were, in part, an outlet for an intense religious piety which rose up in the late 11th century among the lay public. A crusader would, after pronouncing a solemn vow, receive a cross from the hands of the Pope or his legates and was thenceforth considered a "soldier of the Church". This was partly because of the Investiture Controversy, which had started around 1075 and was still on-going during the First Crusade. As both sides of the Investiture Controversy tried to marshal public opinion in their favor, people became personally engaged in a dramatic religious controversy. The result was an awakening of intense Christian piety and public interest in religious affairs, and was further strengthened by religious propaganda, which advocated Just War in order to retake the Holy Land from the Muslims. The Holy Land included Jerusalem (where the death, resurrection and ascension into heaven of Jesus took place) and Antioch (the first Christian city).