An interregnum and a war of succession began in the Kingdom of Portugal in 1383 when King Ferdinand I died leaving no sons and ended when King John I was crowned in 1385 after his victory during the Battle of Aljubarrota.
The Portuguese interpret the era as their earliest national resistance movement to counter Castilian intervention, and Robert Durand considers it as the "great revealer of national consciousness".
Ferdinand I (Portuguese: Fernando; 31 October 1345 – 22 October 1383), sometimes called the Handsome (o Formoso) or occasionally the Inconstant (o Inconstante), was the King of Portugal from 1367 until his death in 1383. He was also briefly made King of Galicia, in 1369 (a claim which he would maintain until 1373). Facing a lack of legitimate male heirs, his death led to the 1383–85 crisis, also known as the Portuguese interregnum.
The Kingdom of Portugal was a monarchy in the western Iberian Peninsula and the predecessor of the modern Portuguese Republic. Existing to various extents between the mid-12th century and the early 20th century, it was also known as the Kingdom of Portugal and the Algarves after 1471, and was the main constituent of the United Kingdom of Portugal, Brazil and the Algarves, which existed between 1815 and 1822. It coexisted with the Portuguese Empire, the realm's overseas colonies.
The nucleus of the Portuguese state was the County of Portugal, established in the 9th century as part of the Reconquista by Vímara Peres, a vassal of the King of Asturias. The county became part of the Kingdom of León in 1097, and the Counts of Portugal established themselves as rulers of an independent kingdom in the 12th century, following the Battle of São Mamede. The kingdom was ruled by the Afonsine Dynasty until the 1383–85 Crisis, after which the monarchy passed to the House of Aviz.
The Battle of Aljubarrota was fought between the Kingdom of Portugal and the Crown of Castile on 14 August 1385. Forces commanded by King John I of Portugal and his Constable Nuno Álvares Pereira, with the support of English allies, opposed the army of King John I of Castile with its Aragonese and French allies, as well as Genoese mercenaries at São Jorge, between the towns of Leiria and Alcobaça, in central Portugal. The result was a decisive victory for the Portuguese, ruling out Castilian ambitions to the Portuguese throne, ending the 1383–85 Crisis and assuring John's position as King of Portugal.
Portuguese independence was safeguarded and a new dynasty, the House of Aviz, was established. Scattered border confrontations with Castilian troops would persist until the death of John I of Castile in 1390, but these posed no real threat to the new dynasty.