Excerpts from Books and Wikipedia
"Konrad of Mazovia offered the Teutonic Knights another opportunity to crusade: they could come into his dukedom and fight against the enemies of Christ who lived around the Vistula. In exchange, he promised them a northern tract of land in his dukedom for their own “in perpetuity . . . and in addition the lands which they might conquer thereafter with the help of God.” . . . In 1226, Honorius III declared the fight against Lithuanian-speakers to be a new crusade, complete with full absolution of sin for those who took part. The Teutonic Knights took some time to organize and assemble themselves, but in 1233 the first invasion of Prussia, across the Vistula river, began. It was the beginning of a war that would last for decades. What began as the “Baltic Crusade” turned into an ugly, bloody, protracted struggle in which (...) “primitive tribes with no common political organization were obliged hopelessly to protect their lives, farms, tribal independence, and religion against the superior might of the west.” . . . For the next fifty years, the Teutonic Knights would lay waste to the lands of the Lithuanians—fighting, perhaps, for Christ, but hoping to gain themselves a kingdom."