The Congress of Arras causes Burgundy to switch sides in the Hundred Years' War.
The Congress of Arras was a diplomatic congregation established at Arras in the summer of 1435 during the Hundred Years' War, between representatives of England, France, and Burgundy. It was the first negotiation since the Treaty of Troyes and replaced the 15 year agreement between Burgundy and England that would have seen the dynasty of Henry V inherit the French crown.
Toward the close of the Hundred Years' War, both the Congress and the subsequent Treaty of Arras represented diplomatic failures for England and major successes for France and led to the expulsion of the English from France.