Third Fernandine War The Third Fernandine War was the last conflict of the Fernandine Wars , and took place between 1381 – 1382 , between the Crown of Castile and the Kingdoms of Portugal and England . When Henry II of Castile ( " Henry of Trastamara " ) died in 1379 , John of Gaunt , 1st Duke of Lancaster claimed their rights of the throne of the Kingdom of Castile , and again found an ally in Ferdinand I of Portugal . Anglo - Portuguese alliance . In 1381 , breaking the 1373 Treaty of Santarem , King Ferdinand I of Portugal decided to attack back to Kingdom of Castile , thus initiating the Third Fernandine War . For this , he signed an alliance with the Kingdom of England , ruled in this time by the young King Richard II of England . John of Gaunt , 1st Duke of Lancaster , also had , since 1371 , claims to the Castilian throne , and saw in this deal a means of enforcement of this cause . In June , the Duke of Lancaster sent an English army ( composed by the famous English archers ) under the command of the Earl of Cambridge to Lisbon in support of the Portuguese troops in a incursion into the Castilian territory . Naval Portuguese offensive . 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 July 1381 , from Lisbon , a Portuguese fleet under the command of João Afonso Teles de Menezes , Count of Barcelos , sailed towards the mouth of the Guadalquivir river , to prevent the passage of the Castilian fleet . At the same time , the Admiral Fernando Sánchez de Tovar sailed from its base , heading out to the Portuguese coasts . The Portuguese fleet was decisively defeated by the fleet of Don Fernando Sánchez de Tovar at the Battle of the Saltes Island , and the Castilian fleet obtained the total control of the Atlantic Ocean . Meanwhile , the English troops disembarked in Lisbon without any problem . Castilian offensive of 1382 . Fedinand I of Portugal , exhausted by war and by the constant defeats at the hands of the Castilians , began to rethink peace with Castile . 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 . Consequences . The Castilians were victorious , and the Crown of Castile gained the military supremacy in the Atlantic Ocean . Under the conditions of peace stipulated that Beatrice of Portugal , the heiress of Ferdinand I of Portugal , married King John I of Castile . This union meant " de facto " annexation of Portugal to the Crown of Castile , but this treaty did not like the Portuguese nobility , and will be the main consequence of the future important crisis of 1383 - 1385 . On October 22 , King Ferdinand died . 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 . Since diplomatic opposition was no longer possible , the party for independence took more drastic measures , starting the 1383 – 1385 Crisis .