Brandenburg-Prussia
Brandenburg and Prussia were unified two generations later. Anna, granddaughter of Albert I and daughter of Duke Albert Frederick (reigned 1568–1618), married her cousin Elector John Sigismund of Brandenburg. Upon the death of Albert Frederick in 1618, who died without male heirs, John Sigismund was granted the right of succession to the Duchy of Prussia, which was still a Polish fief. From this time the Duchy of Prussia was in personal union with the Margraviate of Brandenburg. The resulting state, known as Brandenburg-Prussia, consisted of geographically disconnected territories in Prussia, Brandenburg, and the Rhineland lands of Cleves and Mark.
During the Thirty Years' War, the disconnected Hohenzollern lands were repeatedly marched across by various armies, especially the occupying Swedes. The ineffective and militarily weak Margrave George William (1619–1640) fled from Berlin to Königsberg, the historic capital of the Duchy of Prussia, in 1637. His successor, Frederick William I (1640–1688), reformed the army to defend the lands.
Frederick William I went to Warsaw in 1641 to render homage to King Władysław IV Vasa of Poland for the Duchy of Prussia, which was still held in fief from the Polish crown. In the first phase of the Second Northern War (1654–1660), he took the duchy as a fief from the Swedish king who later granted him full sovereignty in the Treaty of Labiau. In 1657, this grant was renewed by the Polish king in the treaties of Wehlau and Bromberg. With Prussia, the Brandenburg Hohenzollern dynasty now held a territory free of any feudal obligations, which constituted the basis for their later elevation to kings.
Frederick William I became known as the "Great Elector" for his achievements in organizing the electorate, which he accomplished by establishing an absolute monarchy (see absolutism) in Brandenburg-Prussia. Above all, he emphasized the importance of a powerful military to protect the state's disconnected territories, while the Edict of Potsdam opened Brandenburg-Prussia for immigration of Protestant refugees, and he established a bureaucracy to carry out state business efficiently.
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