The state of the Teutonic Order ceased to exist in 1525 when Grandmaster Albert Hohenzollern introduced secularisation, proclaimed the Duchy of Prussia and became a vassal of Sigismund I of Poland. The Prussian line of Hohenzollern was extinct by 1618 with the death of Albert Frederick and the Duchy was inherited by the Brandenburgian line; Prussia simultaneously entered a personal union with the electorate of Brandenburg known as Brandenburg-Prussia, remaining under Polish suzerainty until the Treaty of Oliva in 1660. The throne was inherited by Frederick I of Prussia who wanted to unite the Duchy with Brandenburg and also wanted to proclaim himself king of Prussia and therefore participated in the Russian-initiated Partitions of Poland in which Warmia was annexed by the Kingdom of Prussia, and henceforth became part of the newly established province of East Prussia until 1945. Together with the rest of the Kingdom, the region became part of the North German Confederation, the German Empire, the Weimar Republic, and Nazi Germany. After the end of World War II, both the German as well as the Slavic Masurian population were expelled by the Polish government. In 1914, the province turned into a battlefield, seeing notable battles such as the Battle of Tannenberg as part of the Eastern Front of World War I.