Prussia began as a small territory in what was later called West and East Prussia, which is now Warmia-Masuria of northern Poland, the Kaliningrad exclave of Russia, and the Klaipeda Region of Lithuania. The region was largely populated by Old Prussians, a Baltic people related to the Lithuanians and Latvians.
In 1226 Polish Duke, Konrad I, invited the Teutonic Knights, a German military order of crusading knights headquartered in Acre, to conquer the Baltic tribes on his borders. However, during 60 years of struggles against the Old Prussians, the Teutonic Knights created an independent state which came to control Prussia. The Knights were eventually defeated by Polish troops at Grunwald (1410) and were forced to acknowledge the sovereignty of the Polish king Casimir IV Jagiellon in the Peace of Thorn in 1466, losing western Prussia to Poland in the process.
In 1525 Grand Master Albert I Hohenzollern became a Lutheran Protestant and secularized the Order's remaining Prussian territories into the Duchy of Prussia. For the first time, these lands, the area east of the mouth of the Vistula river were in the hands of a branch of the Hohenzollern family. Furthermore, with the dissolution of the Order, Albert could now marry and produce offspring.
The unification of Brandenberg and Prussia came two generations later. Frederick William went to Warsaw in 1641 to render homage to King Wladyslaw IV Vasa of Poland for the Duchy of Prussia, which was still held in fief from the Polish crown. Taking advantage of the difficult position of Poland vis-a-vis Sweden in the Northern Wars, and his friendly relations with Russia during a series of Russo-Polish wars, Frederick William later managed to obtain a discharge from his obligations as a vassal to the Polish king; he was finally given independent control of Prussia in 1657. It was one of the turning points in the history of Prussia.
In 1701, Frederick William's son, Elector Frederick III, upgraded Prussia from a duchy to a kingdom, and crowned himself King Frederick I. To avoid offending Leopold I, emperor of the Holy Roman Empire where most of his lands lay, Frederick was only allowed to title himself "King in Prussia", not "King of Prussia". However, Brandenburg was treated in practice as part of the Prussian kingdom rather than a separate state.