Born in Wolfenbüttel, Germany, Karl Wilhelm was duke of Brunswick-Lüneburg from 1780 until his death and ruled over the Wolfenbüttel subdivision of the duchy.
Until Valmy and Jena, Karl was considered a master of the warfare of the period. He was also a cultured and benevolent despot in the model of Frederick the Great. He married Augusta, a sister of George III of Great Britain.
After receiving a wide-ranging education, Karl saw his first military action in the North German campaign of 1757 during the Seven Years' War, under Prince William Augustus, Duke of Cumberland. His gallant actions at the Battle of Hastenbeck where he charged at the head of an infantry brigade won him much admiration leading him to choose a military career. He was soon to be recognised as a master of irregular warfare. As for more formal fighting, his action at Minden and Warburg made him appreciated as an excellent subordinate.
At the end of the Seven Years' War, Karl married Augusta, the daughter of Frederick, Prince of Wales (father of the future George III). Becoming Duke of Brunswick in 1780, he was successful (with the assistance of his minister, Feonçe von Rotenkreuz) in rescuing the state from the bankruptcy into which the war had brought it. His reputation as a model sovereign grew because of his success in both the political and military life.
Indeed, he came to be seen as the quintessential form of the 18th-century benevolent despot: although whether he was as wise, economical, prudent and kindly as he has been made out to be is another question. As a Prussian field marshal (1787), he rendered important service to the King of Prussia and was much occupied in the running of his regiment and frequently engaged in diplomatic and other state affairs. In common with the Prussian monarch, Frederick William III, he had a pronounced taste for caution, attempting to keep his duchy from all foreign entanglements. Had it not been for this excessive caution, he might have been another Frederick the Great (in fact Karl's uncle). As an enthusiastic adherent of the Germanic and anti-Austrian policy of Prussia he joined the Fürstenbund, in which, as he now had the reputation of being the best soldier of his time, he was the destined commander-in-chief of the federal army.
Brunswick's 'clean sheet' was significantly tarnished however by his actions during the French Revolution in the early summer of 1792. On France declaring war against Austria, voted on April 20, 1792, the Catholic Holy Roman Emperor Leopold II and the Protestant King of Prussia Frederick William II had combined armies and put them under Brunswick's command.
His first act was to issue the “Brunswick Proclamation” or “Brunswick Manifesto”, given at Coblenz on July 25, 1792, threatening war and ruin to soldiers and civilians alike, should the Republicans injure Louis XVI and his family. Intended to threaten the French public into submission, it had exactly the opposite effect. It helped begin the French Revolutionary Wars. He was less successful against the highly motivated citizen's army that met him at Valmy. Having secured Longwy and Verdun without serious resistance, he unexpectedly found himself heavily outnumbered at Valmy, turned back with a mere skirmish, and evacuated France. When he counterattacked the Revolutionary French who had invaded Germany, in 1793, he recaptured Mainz, but resigned in 1794 in protest at interference by Frederick William II of Prussia.
He returned to command the Prussian army in 1806 (aged 71!) but was routed by Napoleon's marshal Davout at Auerstedt and died of the wounds he received. As commander-in-chief of the main body of the Prussian Army, Brunswick was mortally wounded at the onset of the battle of Auerstadt, while leading a division of reinforcements that had just arrived on the battlefield. Galloping at the head of his troops, he came too close to French sharpshooters and a bullet struck through his left eye. The Duke died three weeks later from his wound.
His successor, Friedrich Wilhelm (1771 – June 16, 1815), who was one of the bitterest opponents of Napoleonic domination in Germany, took part in the war of 1809 at the head of a corps of partisans; fled to England after the Battle of Wagram, and returned to Brunswick in 1813, where he raised fresh troops. He was killed at the Battle of Quatre Bras.
Sources;
Lord Edmond Fitzmaurice, Charles William Ferdinand, Duke of Brunswick: An historical study, 1735-1806, [S.l.] : Longmans, Green, 1901.
Allgemeine deutsche Biographie, vol. ii. (Leipzig, 1882)
Arthur Chuquet, Les Guerres de la Révolution: La Première Invasion prussienne (Paris)