The Catholic kings of Denmark and Sweden, the German Livonian and Teutonic military orders, and their allies launch the Northern Crusades, or Baltic Crusades, against the pagan peoples of Northern Europe around the southern and eastern shores of the Baltic Sea.

Swedish and German campaigns against Russian Eastern Orthodox Christians are also sometimes considered part of the Northern Crusades.

(Some of these wars are called crusades during the Middle Ages, but others, including most of the Swedish ones, will first be dubbed crusades by nineteenth century romantic nationalist historians.)

The official starting point for the Northern Crusades is Pope Celestine III's call in 1193; but the already Christian kingdoms of Scandinavia and the Holy Roman Empire had started to move to subjugate their pagan neighbors even earlier, notably 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 eastern Baltic world is transformed by military conquest: first the Livs, Latgallians and Estonians, then the Semigallians, Curonians, Prussians and the Finns undergo defeat, baptism, military occupation and sometimes extermination by groups of Germans, Danes and Swedes.