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How did Ducal Prussia become united with Brandenburg in 1618?

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Historical map showing the unification of Germany by Prussia in the late 19th century. The map highlights in green the Kingdom of Prussia as of 1866, and in yellow the additional territories that formed the German Empire by 1871-1918, encompassing most of north-central Europe. Neighboring countries such as France, Belgium, the Netherlands, Denmark, Sweden, Russia, Austria-Hungary, and Switzerland are labeled, with major regions and states within the empire clearly marked. — Prussia The unification of Germany by Prussia, which brought most of north-central Europe into one kingdom.

The union of Ducal Prussia with the electorate of Brandenburg in 1618 occurred through dynastic inheritance following the death of Albert Frederick, the duke of Prussia, who died without a male heir.1

The connection was established through the marriage of John Sigismund, the elector of Brandenburg, to Anna, the daughter of Albert Frederick. Because Albert Frederick left no son, the duchy passed to John Sigismund in his capacity as the husband of the duke's eldest daughter. This succession was further solidified by earlier arrangements, including a 1569 agreement between the Hohenzollern elector and the duke of Prussia that had secured the Brandenburg dynasty's right to the succession should the duke's family line become extinct.123

This union was a pivotal development in the rise of the Hohenzollern monarchy, eventually providing the territorial foundation for the later Kingdom of Prussia.1