King in Prussia

Originally the dukes of Prussia held the fief as vassals of the King of Poland, until the Treaties of Labiau (1656) and Bromberg (1657), with which Frederick William, the Great Elector, achieved full sovereignty from the Polish Crown.

Frederick thus argued that German law of the time allowed him to rule Prussia as a kingdom, outside the borders of the Holy Roman Empire.

Hence, even though Brandenburg was still legally part of the empire and ruled in personal union with Prussia, the two states came to be treated as one de facto.

On 17 January 1701, Frederick dedicated the royal coat of arms, the Prussian black eagle with the motto "suum cuique" imprinted.

They were victorious over the Austrian Habsburg monarchy in the three Silesian Wars, greatly increasing their power through the acquisition of Silesia.