Despite being of royal descent, as the younger son of a cadet branch Peter's early prospects were that of a minor noble, with a few scattered fiefs in the Île-de-France and Champagne.

He was initially destined for a career in the clergy, which he later renounced, earning him the nickname Mauclerc (French: mauvais clerc, bad-cleric).

King Philip thus broke off the betrothal of Alix and the Breton lord Henry of Penthièvre, and turned to his French cousin Peter, then in his early twenties.

John chased off some French forces in the north of Poitou, and then moved to the southern edge of Brittany, opposite Nantes.

Peter drove him off after a brief skirmish but did nothing to hinder John's subsequent movement up the Loire valley where he took a few Breton fortresses and then besieged La Roche-aux-Moines.

After the negotiations were completed (in 1218), William Marshal, the regent for the young Henry III of England, recognized Peter as Earl of Richmond, in place of Eleanor of Brittany who remained under English imprisonment.

To that end Peter simply declared new rules by fiat, and then faced the inevitable turmoil that resulted from the reaction of his barons.

The six Breton bishops were the other threat to the ducal power, for they had substantial landholdings (including control of all or part of the few cities in Brittany), and were recalcitrant in the face of Peter's attempts to raise revenues by increasing taxes or simply taking possession of episcopal holdings.

Her death meant that Peter was no longer duke, although he continued to rule the duchy with undiminished authority, as regent for his son John, then a boy of four or so.

Peter helped Philip II's successor, Louis VIII, in his fight against Henry III of England (in the sieges of Niort and La Rochelle in 1224).

However, Pope Gregory IX announced that he only recognized descendants of Peter and Alix legitimate heirs to Brittany; whatever the intention of Henry, Eleanor was after all not restored of any titles nor released from confinement until her death in 1241.

While there, Peter's troops along with some local knights were attacked by heavily armed Mamluk cavalry, firing their bows, but the crusader force managed to outflank and defeat them, taking a few prisoners with them back to Jaffa.

In early November, two days into a march from Acre to Ascalon, Peter and his lieutenant Raoul de Soissons split off from the main force to conduct a raid.

The Muslims were routed and fled inside the castle, where Peter's men followed them, killed many, took some captives, and seized the booty and edible animals of the caravan.

[1] Alix and Peter had three children: His second wife was Marguerite de Montaigu,[7] Lady of Montaigu, Commequiers, La Garnache then Machecoul, and widow of Hugh I de Thouars (died 1230), a brother to Guy of Thouars; this made Marguerite a paternal line aunt of Alix.