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".
The bourgeoisie and the nobility worked together to establish the Avis dynasty, a branch of the Portuguese House of Burgundy, securely on an independent throne. That contrasted with the lengthy civil wars in France (Hundred Years' War) and England (War of the Roses), which had aristocratic factions fighting powerfully against a centralised monarchy.
In Portugal it is sometimes known simply as the Interregnum (or the First Interregnum, if the 1580 Portuguese succession crisis is counted as a "Second Interregnum"), the 1383–1385 crisis () or the Avis Revolution ().
Background
In 1383, King Ferdinand I of Portugal was dying. From his marriage to Leonor Telles de Menezes, only Princess Beatrice of Portugal survived. Her marriage was the major political issue of the day since it would determine the future of the kingdom.
Several political factions lobbied for possible husbands, which included English and French princes. Finally, the king settled for his wife's first choice, King John I of Castile. Ferdinand had waged three wars against Castile during his reign, and the marriage, celebrated in May 1383, was intended to put an end to hostilities by a union of the two crowns but was not a widely accepted solution. The dynastic union meant that Portugal would lose independence to Castile. Many nobles were fiercely opposed to that possibility but were not united under a common pretender to the crown. There were two candidates, both illegitimate half-brothers of Ferdinand: was now the leader of the opposition to the pretensions of John I of Castile, who tried to be recognised as monarch iure uxoris, against the Treaty of Salvaterra. The Castilian forces refused to offer battle, after two months no significant town was taken and the allies, struck by disease and lack of supplies, met with an overwhelming failure.