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 success of the First Crusade inspired 12th-century popes such as CelestineIII, InnocentIII, HonoriusIII, and GregoryIX to call for military campaigns with the aim of Christianising the more remote regions of northern and north-eastern Europe. These campaigns are known as the Northern Crusades. The Wendish Crusade of 1147 saw Saxons, Danes, and Poles attempt to forcibly convert the tribes of Mecklenburg and Lusatia, who were Polabian Slavs or "Wends". CelestineIII called for a Crusade in 1193, but when Bishop Berthold of Hanover responded in 1198, he led a large army into defeat and to his death. In response, InnocentIII issued a bull declaring a Crusade, and Hartwig of Uthlede, Bishop of Bremen, along with the Brothers of the Sword brought all of the north-east Baltic under Catholic control. Konrad of Masovia gave Chelmno to the Teutonic Knights in 1226 as a base for a Crusade against the local Polish princes. The Livonian Brothers of the Sword were defeated by the Lithuanians, so in 1237 GregoryIX merged the remainder of the order into the Teutonic Order as the Livonian Order. By the middle of the century, the Teutonic Knights completed their conquest of the Prussians before conquering and converting the Lithuanians in the subsequent decades. The order also came into conflict with the Eastern Orthodox Church of the Pskov and Novgorod Republics. In 1240 the Orthodox Novgorod army defeated the Catholic Swedes in the Battle of the Neva, and, two years later, they defeated the Livonian Order in the Battle on the Ice.