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.
Crusades
In 1095, at the Council of Piacenza, Byzantine Emperor Alexios I Komnenos requested military aid from Pope Urban II, probably in the form of a small body of mercenary reinforcements he could direct and control. Alexios had restored the Empire's finances and authority, but he still faced a number of foreign enemies, particularly the migrating Turks who had colonised the sparsely populated areas of Anatolia. At the Council of Clermont later that year, Urban raised the issue again and preached for a Crusade. Many historians consider that Urban also hoped that aiding the Eastern Church would lead to its reunion with the Western under his leadership.
Almost immediately Peter the Hermit led thousands of mostly poor Christians out of Europe in what became known as the People's Crusade. He claimed he had a letter from heaven instructing Christians to prepare for the imminent apocalypse by seizing Jerusalem. The motivations of this Crusade included a "messianism of the poor" inspired by an expected mass ascension into heaven at Jerusalem. Germany witnessed the first incidents of major violent European antisemitism when these Crusaders massacred Jewish communities in what became known as the Rhineland massacres. In Speyer, Worms, Mainz, and Cologne the range of anti-Jewish activity was broad, extending from limited, spontaneous violence to full-scale military attacks. The Crusaders journeyed, despite advice from Alexios' to wait for the nobles, to Nicaea. Only 3000 survived an ambush by the Turks at the Civetot.