information:highlighting, structural:summarization
Brandenburg-Prussia

Brandenburg and Prussia united two generations later. In 1594 Duchess Anna of Prussia, granddaughter of Albert I and daughter of Albert Frederick, Duke of Prussia (reigned 1568–1618), married her cousin Elector John Sigismund of Brandenburg. When Albert Frederick died in 1618 without male heirs, John Sigismund was granted the right of succession to the Duchy of Prussia, then 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 (1618–1648), various armies repeatedly marched across the disconnected Hohenzollern lands, especially the occupying Swedes. The ineffective and militarily weak Elector 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 January 1656, during the first phase of the Second Northern War (1654–1660), he received the duchy as a fief from the Swedish king who later granted him full sovereignty in the Treaty of Labiau (November 1656). In 1657 the Polish king renewed this grant 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 succeeded in organizing the electorate by establishing an absolute monarchy in Brandenburg-Prussia, an achievement for which he became known as the "Great Elector". Above all, he emphasised the importance of a powerful military to protect the state's disconnected territories, while the Edict of Potsdam (1685) opened Brandenburg-Prussia for the immigration of Protestant refugees (especially Huguenots), and he established a bureaucracy to carry out state administration efficiently.