The Northern Crusades or Baltic Crusades were Christian colonization and Christianization campaigns undertaken by Catholic Christian military orders and kingdoms, primarily against the pagan Baltic, Finnic and West Slavic peoples around the southern and eastern shores of the Baltic Sea, and also against Orthodox Christian Slavs (East Slavs).
The most notable campaigns were the Livonian and Prussian crusades. Some of these wars were called crusades during the Middle Ages, but others, including most of the Swedish ones, were first dubbed crusades by 19th-century romantic nationalist historians. However, crusades against Estonians, but also against 'other pagans in those parts' were authorized by Pope Alexander III in the bull Non parum animus noster, in 1171 or 1172.
At the outset of the northern crusades, Christian monarchs across northern Europe commissioned forays into territories that comprise modern-day Estonia, Finland, Latvia, Lithuania, Poland, and Russia. The indigenous populations of Pagans suffered forced baptisms and the ravages of military occupation. Spearheading, but by no means monopolizing these incursions, the ascendant Teutonic Order profited immensely from the crusades, as did German merchants who fanned out along trading routes traversing the Baltic frontier.
The official starting point for the Northern Crusades was Pope Celestine III's call in 1195, but the Catholic kingdoms of Scandinavia, Poland and the Holy Roman Empire had begun moving to subjugate their pagan neighbors even earlier. The non-Christian people who were objects of the campaigns at various dates included:
the Polabian Wends, Sorbs, and Obotrites between the Elbe and Oder rivers (by the Saxons, Danes, and Poles, beginning with the Wendish Crusade in 1147)
the Finns proper in the 1150s in the First Crusade by the Swedes; by the Danes in 1191 and 1202; Tavastia in 1249 in the Second Crusade by the Swedes; and Karelia in 1293 in the Third Crusade by the Swedes; Christianization in these areas had started earlier.