Frederick William, duke of Brunswick (1771-1815), was one of the most implacable enemies of Napoleonic France, and became known as the 'Black Duke'.
Frederick William was the fourth son of Duke Karl Wilhelm Ferdinand and the daughter of Frederick, Prince of Wales. He inherited his duchy in 1806, after his father was mortally wounded at the battle of Auerstädt (14 October 1806), but he wasn't left in possession of it for long. In 1807, after the Russians had been forced to come to terms with the French, Brunswick was seized under the terms of the Treaty of Tilset and used to create the Kingdom of Westphalia, which was then given to Jérome Bonaparte.
In 1809 the exiled Duke formed the 'Black Legion', a military unit that served in the Austrian army. The Legion dressed in black, and used the death's head insignia. Although the Duke was fiercely anti-French, his former subjects don’t appear to have been so concerned, and very few Westphalian officers or men joined the legion. Instead Brunswick had to recruit most of his men in Bohemia, then part of the Austrian empire.
The legion fought in the Franco-Austrian War of 1809. It was originally part of a force led by General Carl Friedrich Freiherr on Am Ende, which occupied Leipzig, and then became part of IX Corps, under General Michale Freiherr von Keinmayer. Brunswick's men occupied Dresden, where their presence greatly annoyed Napoleon, not least because his brother Jérome didn't make much of an effort to expel him.
After some initial Austrian successes, the war ended as yet another French victory. In order to keep fighting, Brunswick marched his troops across Germany. After storming Halberstadt, the duke moved on to Brunswick, before moving north to Elsfleth on the Weser, where he was evacuated by the Royal Navy and landed on the Isle of Wight.
The Legion entered British service, and was sent to Spain to serve under Wellington. Brunswick himself didn't accompany the Legion into the field, probably greatly to Wellington's relief!
As Germany rose up against France in 1813, the duke was restored to his Duchy. He formed a new Brunswick army, which he led in the Waterloo campaign. This force was a mix of new recruits and veterans from Spain, and used the current Prussian drill regulations. The duke was killed while rallying his troops at the battle of Quatre Bras. He had been ordered to support the Dutch, and his army came under heavy artillery fire. After a failed cavalry charge the Duke attempted to rally his troops and was shot through the liver.
Frederick William was succeeded by his young son Karl II, who was a minor at the time of his succession. Until he came of age the Duchy was ruled by a regency led by the British Prince Regent.