Arthur III (Breton: Arzhur), more commonly known as Arthur de Richemont (24 August 1393 – 26 December 1458), was briefly Duke of Brittany from 1457 until his death. He is noted primarily, however, for his role as a leading military commander during the Hundred Years' War. Although Richemont briefly sided with the English once, he otherwise remained firmly committed to the House of Valois. He fought alongside Joan of Arc, and was appointed Constable of France. His military and administrative reforms in the French state were an important factor in assuring the final defeat of the English in the Hundred Years' War.