How a desperate Portuguese revolution secured independence against a massive Castilian invasion force
When King Ferdinand I died without a male heir in 1383, Portugal faced total absorption by Castile. A grassroots resistance movement, led by the Grandmaster of Avis, defied the odds to crown a new king and secure the nation's sovereignty through a series of improbable military victories.
The 1383–1385 crisis, often called the Avis Revolution, began as a succession struggle following the death of King Ferdinand I. With his only child, Princess Beatrice, married to John I of Castile, the kingdom faced the imminent loss of its independence. While the dowager queen Leonor assumed the regency, the merchant class and traditional nobility in Lisbon and other municipalities rejected the union, rallying instead behind John, Grandmaster of Avis, an illegitimate half-brother of the late king.
The conflict escalated into a full-scale war, characterized by the Castilian siege of Lisbon and the strategic brilliance of the Portuguese Constable, Nuno Álvares Pereira. Despite being vastly outnumbered, the Portuguese forces utilized tactics similar to those seen in the Hundred Years' War, employing longbowmen and defensive structures to neutralize superior cavalry. This culminated in the decisive Battle of Aljubarrota on 14 August 1385, where a force of 6,500 Portuguese and English troops annihilated an invading army of about 32,000 men.
The victory at Aljubarrota solidified John of Avis as King John I, ending the interregnum and establishing the Avis dynasty. This era also birthed the Anglo-Portuguese Alliance, formalized by the 1386 Treaty of Windsor. This pact, which remains one of the oldest in history, provided a framework for mutual support that influenced centuries of European geopolitics, from the expulsion of Spanish Habsburgs in 1640 to the strategic use of the Azores during the Second World War.
Source: 1383–1385 Portuguese interregnum
