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.
Ayyubid dynasty
Pope Gregory VIII called for a Third Crusade against the Muslims in early 1189. Frederick Barbarossa of the Holy Roman Empire, Philip Augustus of France, and Richard the Lionheart of England formed an alliance to reconquer Jerusalem. Meanwhile, the Crusaders and the Ayyubids fought near Acre that year and were joined by the reinforcements from Europe. From 1189 to 1191, Acre was besieged by the Crusaders, and despite initial Muslim successes, it fell to Richard's forces. A massacre of 2,700 Muslim inhabitants ensued, and the Crusaders then planned to take Ascalon in the south.