Often overlooked in considerations of the Crusades were the “Northern Crusades” – invasions of the various Baltic regions of northeastern Europe (i.e. parts of Denmark, northern Germany, Latvia, Estonia, Lithuania, and Finland) between 1171, when the Pope Alexander III authorized a crusade against the heathens of the east Baltic, and the early fifteenth century, when the converted kingdoms and territories of the Baltic began to seize independence from their crusading overlords: the Teutonic Knights.
The Teutonic Knights were a knightly order founded during the Third Crusade at a hospital in the Latin city of Acre. They were closely modeled after the Templars, adopting their “rule” (their code of conduct) and spending most of the twelfth century crusading in the Holy Land. Their focus shifted, however, in the middle of the century when they began leading Crusades against the pagan peoples of the eastern Baltic, including the Lithuanians, Estonians, Finns, and various other groups.
The Baltic lands were the last major region of Europe to remain pagan. Neither Latin nor Orthodox missionaries had made significant headway in converting the people of the region, outside of the border region between the lands of the Rus and the Baltic Sea. Thus, the Teutonic Knights could make a very plausible case for their Crusades as analogous to the Spanish Reconquest, and the Teutonic Knights proved very savvy at placing agents in the papal court that worked to maintain papal support for their efforts.
The Teutonic Order ultimately outlasted the other crusading orders by centuries. The Order was very successful at drumming up support from European princes and knights, relying on annual expeditions of visiting warriors to do most of the fighting while the Teutonic Knights themselves literally held down the fort in newly-built castles. They were authorized by various popes not only to conquer and convert, but to rule over the peoples of the east Baltic, and thus by the thirteenth century the Teutonic Knights were in the process of conquering and ruling Prussia, parts of Estonia, and a region of southeastern Finland and present-day Lithuania called Livonia. These kingdoms lasted a remarkably long time; the Teutonic Order ruled Livonia all the way until 1561, when it was finally ousted. Thus, for several centuries, the map of Europe included the strange spectacle of a theocratic state: one ruled directly by monk-knights, with no king, prince, or lord above them.
The Northern Crusades were, in some ways, as important as the Crusades to the Holy Land in that they were responsible for extinguishing the last remnants of paganism in Europe – it was truly gone by the late fourteenth century in Lithuania, Estonia, and Livonia – and in conquering a large territory that would one day be a core part of Germany itself: Prussia.