[4] In 1381, breaking the 1373 Treaty of Santarem, King Ferdinand I of Portugal decided to attack Castile, thus initiating the Third Fernandine War.

[4] To prevent the English contingent being intercepted at sea by the navy of Castile, the Portuguese monarch planned a naval offensive against the Castilian fleet, anchored in Seville.

In 1382, the Castilians, led by the King John I of Castile and the Admiral Fernando Sánchez de Tovar, with a great and vigorous offensive by sea and land, came to the gates of Lisbon, finally forcing the King of Portugal to sign a favourable peace by the Castilians in August, with John I, by the Treaty of Elvas of 1382.

This union meant de facto annexation of Portugal to the Crown of Castile, but this treaty did not please the Portuguese nobility, and became the main cause of the future important crisis of 1383-1385.

According to the marriage contract, dowager Queen Leonor Telles de Menezes assumed regency in the name of her daughter Beatrice and son-in-law, John I of Castile.