Treaty of Troyes
When Henry V invaded France, John was initially allied to the French. However, he missed the Battle of Agincourt. His brother Arthur de Richemont participated, though, and was captured and imprisoned by the English. The confusion in the aftermath of the battle allowed John to seize Saint-Malo which had been annexed by the French. He then adopted a policy of switching between the two parties, English and French. He signed the Treaty of Troyes, which made Henry V heir to France, but he allowed his brother Arthur de Richemont to fight for the French.