The Duchy of Prussia was a historical territory located in northeastern Europe, established in 1525. It originated from the Teutonic Knights' state and became a duchy when the Grand Master of the Knights, Albert of Hohenzollern, converted to Lutheranism and secularized the order's lands. The duchy was a vassal of the Kingdom of Poland until it was elevated to a kingdom in 1701. In 1618, the Duchy of Prussia entered a personal union with the Margraviate of Brandenburg, leading to the formation of the Brandenburg-Prussia state. This union played a significant role in the rise of the Kingdom of Prussia, which became a major European power in the 18th and 19th centuries.