Upon the invitation of Duke Konrad I of Masovia, the Teutonic Knights took possession of Prussia in the 13th century and created a monastic state to administer the conquered Old Prussians. Local Old-Prussian (north) and Polish (south) toponyms were gradually Germanised. The Knights' expansionist policies, including occupation of Polish Pomerania with Gdańsk/Danzig and western Lithuania, brought them into conflict with the Kingdom of Poland and embroiled them in several wars, culminating in the Polish-Lithuanian-Teutonic War, whereby the united armies of Poland and Lithuania, defeated the Teutonic Order at the Battle of Grunwald (Tannenberg) in 1410. Its defeat was formalised in the Second Treaty of Thorn in 1466 ending the Thirteen Years' War, and leaving the former Polish region Pomerania/Pomerelia under Polish control. Together with Warmia it formed the province of Royal Prussia. Eastern Prussia remained under the Knights, but as a fief of Poland. 1466 and 1525 arrangements by kings of Poland were not verified by the Holy Roman Empire as well as the previous gains of the Teutonic Knights were not verified.

Which group took possession of Prussia in the 13th century?