In 1443, after the death of his cousin
Infante Diogo and by the instigation of his father, Peter was elected the
Constable of Portugal, enraging
Afonso, Marquis of Valença, the son of his half-uncle
Afonso, 1st Duke of Braganza, who believed the title was rightfully his. This political move, along with Afonso V's marriage to
Isabella of Coimbra, Peter's sister, instead of the granddaughter of the Duke of Braganza, caused significant conflict between the two parties. In the same year, after the death of his uncle
Ferdinand, Peter was granted the mastership of the order of Avis. In 1448, when King
Afonso V of Portugal became of age and took reign, the Duke of Coimbra stepped down as regent. Afonso V, because of influence of the Duke of Braganza, who became the political enemy of Peter's father, voided all laws created under Coimbra's regency. Peter was stripped of both the constableship and the master of Avis. Intense disagreement and intrigue led to the
Battle of Alfarrobeira, culminating in the death of Pedro and the exile of his children. Unlike his siblings, who went to the court of their aunt
Isabella in Burgundy, Peter fled to
Castile. Peter's exile in Castile constitutes the main period of his activity as a writer. Described as a disciple of
Rodríguez del Padrón, he completed two projects,
Satira de Infelice e Felice Vida and
Tragédia de la insigne reina doña Isabel. The works are considered some of the earliest examples of sentimental fiction. In 1456 or 1457, Peter reconciled with King Afonso V and the Duke of Braganza, thus allowing him to return to Portugal and reclaim all his family's previous possessions. Upon his return, he participated in several of Afonso's expeditions to northern Africa. == King of Aragon ==