Frederick William , Duke of Brunswick - Wolfenbüttel 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 Napoleonic domination in Germany . He briefly ruled the state of Brunswick - Wolfenbüttel from 1806 - 1807 . Life . Prince Frederick William of Brunswick - Wolfenbüttel was born in Braunschweig as the fourth son of Charles William Ferdinand , Duke of Brunswick - Lüneburg , and Princess Augusta of Great Britain . He was the 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 a local folk hero . He then fled to England to join forces with his brother - in - law , later to be King George IV . His troops were taken into British pay and the Duke was granted the rank of lieutenant general in the British Army on 1 July 1809 . His corps of originally 2 , 300 soldiers was largely destroyed in battles in Spain and Portugal during the Peninsular War . Frederick William returned to Braunschweig in December 1813 , after Prussia had ended French domination in Braunschweig - Lüneburg . When Napoleon returned to the political scene in 1815 during the Hundred Days , Frederick William raised fresh troops . He was killed by a gunshot at the Battle of Quatre Bras on 16 June . The night before he had attended the Duchess of Richmond ' s ball in Brussels and left it happy to have a chance to show his fighting ability . Family . On 1 November 1802 , in Karlsruhe , Frederick William married Princess Marie Elisabeth of Baden ( 7 September 1782 Karlsruhe – 20 April 1808 Bruchsal ) , daughter of Charles Louis , Hereditary Prince of Baden . The couple had three children before Marie died of puerperal fever four days after giving birth to a stillborn daughter .