Frederick William, Duke of Brunswick-Wolfenbüttel (; 9 October 1771 – 16 June 1815), was a German prince and Duke of Brunswick-Lüneburg and Oels. Nicknamed "The Black Duke", he was a military officer who led the Black Brunswickers against French domination in Germany. He briefly ruled the state of Brunswick-Wolfenbüttel from 1806 to 1807 and again from 1813 to 1815. Life Prince Frederick William of Brunswick-Wolfenbüttel was born in Braunschweig as the fourth son of Charles William Ferdinand, Duke of Brunswick-Lüneburg (on the latter's 36th birthday), and Princess Augusta of Great Britain. He was the first cousin and brother-in-law (from 8 April 1795) of his friend George IV, Prince Regent of the United Kingdom (from 1811). He joined the Prussian army in 1789 as a captain and participated in battles against Revolutionary France. In 1805, after his uncle, Frederick Augustus, Duke of Oels, had died childless, Frederick William inherited the Duchy of Oels, a small mediatized principality in Silesia subordinate to the King of Prussia. In October 1806, Frederick William participated in the Battle of Jena-Auerstädt as a major general of the Prussian army, of which his father was the field marshal. His father died from a wound he received in this battle, and Frederick William inherited Brunswick-Wolfenbüttel, since his eldest brother had died childless two months earlier, and both the second and third brother were mentally retarded. After the defeat of Prussia in the Fourth Coalition, his state remained under the control of France, however, and was formally made a part of the short-lived Napoleonic Kingdom of Westphalia in 1807. Frederick William fled to his parents-in-law in Bruchsal in the Grand Duchy of Baden, which had remained a sovereign state with the dissolution of the Holy Roman Empire in 1806 by Francis II, where he lived for the next few years. When the War of the Fifth Coalition broke out in 1809, Frederick William used this opportunity to create a corps of partisans with the support of the Austrian Empire. This corps was called the Black Brunswickers because they wore black uniforms in mourning for their occupied country. He financed the corps independently by mortgaging his principality in Oels, and made his way from Austrian Bohemia through the French-allied states of Saxony and Westphalia to the North Sea coast. Frederick William briefly managed to retake control of the city of Braunschweig in August 1809, which gained him the status of