Frederick William, Duke of Brunswick-Wolfenbüttel lost his father in battle and his duchy to Napoleon Bonaparte who incorporated it in his Kingdom of Westphalia in 1807, ruled by his brother Jérôme Bonaparte. In exile in Austrian controlled Bohemia, Duke Frederick raised a volunteer force or "freikorps" to fight the French and their German allies. The volunteers were equipped by the Austrian Empire; the main colour of their uniforms was black, giving rise to the epithet of "Schwarze Schar" ("Black Horde") but were known in English as the "Black Brunswickers". The corps campaigned with the Austrian Army in the summer of 1809, but when the Austrians concluded the Armistice of Znaim on 12 July, the duke refused to be bound by it and declared his intention to join with the forces of the United Kingdom, the only power of the Fifth Coalition who were still fighting Napoleon.