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
The two-century attempt to recover the Holy Land ended in failure. Following the First Crusade there were six major Crusades and numerous less significant ones. After the last Catholic outposts fell in 1291, there were no more Crusades; but the gains were longer lasting in Northern and Western Europe. The Wendish Crusade and those of the Archbishop of Bremen brought all the North-East Baltic and the tribes of Mecklenburg and Lusatia under Catholic control in the late 12th century. In the early 13th century the Teutonic Order created a Crusader state in Prussia and the French monarchy used the Albigensian Crusade to extend the kingdom to the Mediterranean Sea. The rise of the Ottoman Empire in the late 14th century prompted a Catholic response which led to further defeats at Nicopolis in 1396 and Varna in 1444. Catholic Europe was in chaos and the final pivot of Christian–Islamic relations was marked by two seismic events: the fall of Constantinople to the Ottomans in 1453 and a final conclusive victory for the Spanish over the Moors with the conquest of Granada in 1492. The idea of Crusading continued, not least in the form of the Knights Hospitaller, until the end of the 18th-century but the focus of Western European interest moved to the New World.