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.
Northern Crusades
The Northern Crusades provided a rationale for the growth and expansion of the Teutonic Order of German crusading knights which had been founded in Palestine at the end of the 12th century. Due to Muslim successes in the Holy Land, the Order sought new missions in Europe. Duke Konrad I of Masovia in west-central Poland appealed to the Knights to defend his borders and subdue the pagan Baltic Prussians in 1226. After the subjugation of the Prussians, the Teutonic Knights fought against the Grand Duchy of Lithuania.