Frederick William, 1771–1815, duke of Brunswick, German military hero. On the death (1806) of his father, Charles William Ferdinand, his duchy was seized by Napoleon I and added to the kingdom of Westphalia. He attempted to liberate his duchy from French control in 1809, when Austria reopened war against France. Frederick William formed a free corps, the "Black Brunswickers," and in a dashing foray advanced through Germany and captured Brunswick. He soon was driven out but succeeded in fleeing with his troops to England. Returning in 1813, he took possession of Brunswick but was killed at Quatre Bras in the Waterloo campaign.
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