John I of Castile : biography
Although most of the Portuguese aristocracy was loyal to his cause, King John I of Castile did not repeat the Castilian successes of the earlier Fernandine Wars (Guerras Fernandinas) and failed to win Coimbra and Lisbon. On 3 September 1384, he left garrisons manned by his supporters among the people, and returned to Castile and sought help from the King of France. Meanwhile, the Master of Aviz tried to seize those places loyal to his adversaries, and even took Almada and Alenquer, but failed to take Cintra, Torres-Torres Novas and Velhas. In March 1385 he went to Coimbra, to which he had summoned the Portuguese Cortes; they declared Beatrice illegitimate and proclaimed the Master of Aviz to be King of Portugal as John I on 6 April. This was in effect a declaration of war against Castile and its claims to the Portuguese throne. Recovering from his recent defeats, the new monarch began his campaign to regain the northern kingdom, and took Viana do Castelo, Braga and Guimarães.
John I of Castile, accompanied by allied French cavalry, then entered Portugal again by way of Ciudad Rodrigo and Celorico to conquer Lisbon and remove John I from the Portuguese throne, but the disastrous defeats suffered by his army in Trancoso and at the Battle of Aljubarrota in May and August 1385 ended any possibility of his reigning as king of Portugal. He fled to Santarém and from there down the Tagus to meet the fleet near Lisbon. In September the Spanish fleet returned to Castile, and John I of Portugal gained control of the places formerly occupied by his adversaries. Advancing from Santarém, he seized the region north of the Duero whose knights had remained faithful to Beatrice and John I of Castile: Villareal Pavões, Chaves and Bragança capitulated at the end of March 1386, and Almeida in early June 1386.
Queen Beatrice had no children with her husband John I of Castile, although a son called Miguel is mentioned in several genealogies written much later and even in some modern history books. There is no contemporary document mentioning him, and his supposed mother was only 10 or 11 years old at his reputed birth. It is most probably a confusion with a grandchild of the Catholic Monarchs who was called Miguel da Paz.