The Fernandine Wars (from the Portuguese Guerras Fernandinas) were a series of three conflicts (1369–70, 1372–73, 1381–82) between the Kingdom of Portugal under King Ferdinand I and the Crown of Castile under Kings Henry II and later John I. They were fought over Ferdinand's claim to the Castilian succession after the murder of King Peter of Castile in 1369. After signing the Treaty of Salvaterra there was a period of civil war (April 1383 - August 1385 Crisis) (1383 - 1385 Portuguese interregnum) in Portugal without a ruling king. This was part of the Hundred Years' War.
The Treaty of Salvaterra de Magos was an agreement signed in 1383 between Portugal and Castile in order to end a period known in Portugal as the Fernandine Wars (after the name of Fernando I).
To celebrate the peace between the two kingdoms, king Fernando I of Portugal agreed on the marriage of his daughter and heir, Beatrice of Portugal, with king John I of Castile. However, to avoid the union of the two countries, this pre-nuptial agreement established the rules to the succession for the two thrones.
It was never Fernando's intention to form a union of the kingdoms of Portugal and Castile. Therefore, it was agreed that following his death his wife, queen Leonor Telles de Menezes would assume the regency in Portugal until the son of Beatrice and John would reach 14 years old, when he would become the future king of Portugal.
The main clauses of the Treaty of Salvaterra stipulated:
In the famous Cortes of Coimbra (1385), João das Regras, a lawyer supporting John Master of Aviz claims to the throne of Portugal, defended that due to a breach of the Treaty of Salvaterra, Beatrice and John I of Castile should be barred from ascending the Portuguese throne.