While he was Count of Guingamp, he fought against the English in Normandy in 1449 and in 1450 with his brother, Francis I, Duke of Brittany, and his uncle the Constable de Richemont.

Since Francis did not have a son, according to the provisions of the first Treaty of Guerande (1365) that did not allow the succession of girls, he appointed Peter in preference to his own daughters, Margaret and Marie, to succeed him.

To seal the marriage, the Duke summoned the Estates of Brittany, a sovereign court, at Vannes to meet on November 13, 1455, in the upper room of la Cohue.

The wedding started on November 16 with a grand mass in Saint Peter's cathedral in Vannes, presided over by the Bishop of Nantes, Guillaume de Malestroit.

His contemporaries described Peter II as simple, well advised by his wife, but little suited to the ducal function, heavy mind as body, prone to mood swings.