Frederick William (German: Friedrich Wilhelm; 16 February 1620 – 29 April 1688) was Elector of Brandenburg and Duke of Prussia, thus ruler of Brandenburg-Prussia, from 1640 until his death in 1688.

This ended with the treaties of Labiau, Wehlau, Bromberg and Oliva; these changed the status of Ducal Prussia from that of a Polish fief to fully sovereign (after a brief period of control by Sweden).

Although he rejoined the anti-French alliance in 1674, this left him diplomatically isolated; despite conquering much of Swedish Pomerania during the Scanian War, he was obliged to return most of it to Sweden in the 1679 Treaty of Saint-Germain-en-Laye.

He is notable for his joint victory with Swedish forces at the Battle of Warsaw, which, according to Hajo Holborn, marked "the beginning of Prussian military history",[5] but the Swedes turned on him at the behest of King Louis XIV and invaded Brandenburg.

After marching 250 kilometres in 15 days back to Brandenburg, he caught the Swedes by surprise and managed to defeat them on the field at the Battle of Fehrbellin, destroying the myth of Swedish military invincibility.

[6] Since his capital Berlin had suffered greatly from the Swedish occupation during the Thirty Years' War, Friedrich Wilhelm commissioned the master engineer Johann Gregor Memhardt to plan a city fortification.

His great grandson, Frederic II, later said of him with regard to the battle: ”He was praised by his enemies, blessed by his people; and posterity dates from that famous day the subsequent elevation of the house of Brandeburg”.

He united the multiple separate domains that his family had acquired primarily by marriage over the decades, and built the powerful unified state of Prussia out of them.

Funding the military through heavy taxes required building up new industry, such as wool, cotton, linen, lace, soap, paper, and iron.

[9] In 1682, at the suggestion of the Dutch merchant and privateer Benjamin Raule, he granted a charter to the Brandenburg Africa Company (BAC), marking the first organised and sustained attempt by a German state to take part in the Atlantic slave trade.

As Brandenburg-Prussia remained economically impoverished after the Thirty Years War, he hoped to replicate the mercantile successes of the Dutch East India Company.

Luise Henriette also distinguished herself through charity, Sophie Dorothea through extraordinary business acumen, which allowed her to increase both her own fortune (and thus the inheritance of her children) and to strengthen the state economy.