Wielding their swords in the name of their faith, the crusaders originally set out to reclaim Jerusalem and its surrounding territory in the Middle East. Increasingly, however, Eastern Europe and the last remaining bastions of pagan Europe became the targets of their religious zeal.

The era officially began in 1147, when the Saxons, Danes, and Poles, responding to Pope Eugene III's call, initiated a crusade against the Wends of the Southern Baltic. This was followed by crusades against the Livonians, Estonians, Finns, Prussians, and Lithuanians. By the 13th century much of the...