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.
Middle Ages
Popes called for crusades to take place elsewhere besides the Holy Land: in Spain, southern France, and along the Baltic. The Spanish crusades became fused with the "Reconquista" of Spain from the Muslims. Although the Templars and Hospitallers took part in the Spanish crusades, similar Spanish military religious orders were founded, most of which had become part of the two main orders of Calatrava and Santiago by the beginning of the 12th century. Northern Europe also remained outside Christian influence until the 11th century or later, and became a crusading venue as part of the Northern Crusades of the 12th to 14th centuries. These crusades also spawned a military order, the Order of the Sword Brothers. Another order, the Teutonic Knights, although founded in the crusader states, focused much of its activity in the Baltic after 1225, and in 1309 moved its headquarters to Marienburg in Prussia.