Answer
Castlereagh resigned in 1809 after fighting a duel with his political rival George Canning. The duel was the result of a dispute over the war with France. Canning had been advocating for a more aggressive approach to the war, while Castlereagh had been more cautious. The duel took place in Belgium, and Castlereagh was wounded in the shoulder. He returned to England and resigned from his position as Foreign Secretary.