Old Prussians
The duchy of the Polans under Mieszko I, predecessor of the Kingdom of Poland, first attempted to conquer and baptize the Baltic tribes during the 10th century, yet repeatedly encountered strong resistance. Not until the 13th century were the Old Prussians subjugated and their lands conquered by the Teutonic Order. The remaining Old Prussians were assimilated during the following two centuries. The old Prussian language, largely undocumented, was effectively extinct by the 17th century.