1383–1385 Portuguese Interregnum

The crisis of succession that ignited the Battle of Aljubarrota and the wider conflict of 1383–1385 was triggered directly by the death of King Ferdinand I of Portugal in October 1383^[1]. Ferdinand died without male issue, leaving as his heir his daughter Beatriz, who had been married in 1383 to King John I of Castile. The terms of that marriage treaty provided that the Portuguese and Castilian crowns would not be formally united, but they offered few practical guarantees of Portuguese autonomy. When Ferdinand died, John I of Castile moved to assert his wife's claim and, by extension, his own influence over the kingdom, precipitating an immediate contest over who would govern Portugal.

The constitutional and dynastic structure of the Iberian peninsula made such a contest almost inevitable. The Castilian and Portuguese royal lines had intermarried for generations, producing overlapping inheritance claims across the region's kingdoms. Ferdinand's own reign had already seen a series of shifting alliances with both Castile and England — culminating in the Treaty of Windsor in 1373 with England and subsequent treaty entanglements with Castile — that reflected Portugal's precarious position between larger powers^[1]. The marriage of Beatriz was itself the product of these pressures, an attempt to stabilize relations with Castile that ended by subordinating Portuguese dynastic independence to Castilian ambition.

The social and political character of the resistance that coalesced around the claimant John, Master of Aviz and illegitimate son of King Peter I, cannot be reduced to dynastic preference alone^[1]. The urban mercantile classes of Lisbon and other coastal towns had material reasons to resist Castilian absorption: trade privileges, municipal charters, and commercial relationships with England — reinforced by the Anglo-Portuguese alliance — all stood to be compromised under Castilian dominance. When the regency council, dominated by pro-Castilian Portuguese nobles, showed itself unable or unwilling to resist John I of Castile's intervention, Lisbon's population backed John of Aviz in a popular uprising in December 1383, during which the Count of Ourém, a leading pro-Castilian magnate, was killed.

The nobility of Portugal was itself divided along lines that partly reflected regional and factional interests rather than pure dynastic loyalty. A significant portion of the titled aristocracy favored acceptance of Beatriz's claim and Castilian suzerainty, calculating that their own positions were best secured under an arrangement that preserved aristocratic privilege even at the cost of an independent crown. Against them stood a coalition of lesser nobles, the Lisbon bourgeoisie, and the Aviz faction, which increasingly framed its cause in terms of Portuguese independence and the legitimacy of native rule. The Portuguese clergy and the Cortes were themselves divided, and it was only the military decision of 1385 that settled the question the political institutions could not resolve^[1].

The decisive Portuguese victory at Aljubarrota in August 1385 effectively ended Castilian military ambitions in Portugal and resolved the succession crisis that had paralyzed the kingdom since the death of Ferdinand I two years earlier^[1]. John of Avis, already acclaimed king by the Cortes of Coimbra earlier that year, was confirmed as John I — the first monarch of the House of Avis — a dynastic settlement that would endure for nearly two centuries. The Castilian claim, pressed through Queen Consort Beatrice and her father John I of Castile, was abandoned in practice following the rout at Aljubarrota, though a formal peace treaty between Portugal and Castile was not concluded until 1411, meaning a generation of low-level frontier tension persisted even after the succession question was settled^[1].

The political reordering within Portugal was as significant as the external military outcome. The old Leonese-descended nobility had largely backed the Castilian candidate; their defeat and subsequent dispossession opened space for a new aristocracy drawn from the lesser nobility and the burghers who had rallied to John of Avis. The Cortes of Coimbra's role in electing John I marked a moment of constitutional importance, elevating the political standing of the urban commons and the middling orders, even if the practical powers of the Cortes contracted again in subsequent reigns^[1]. The war thus accelerated a recomposition of the Portuguese ruling class rather than simply confirming existing hierarchies.

The geopolitical consequences extended to Portugal's relationship with England. The alliance sealed by the Treaty of Windsor in 1386 — negotiated in the immediate aftermath of Aljubarrota and reinforced by John I's marriage to Philippa of Lancaster, daughter of John of Gaunt — became one of the oldest formal alliances in European history and gave Portugal a reliable counterweight to Castilian pressure for generations^[1]. English archers had fought at Aljubarrota, and the treaty transformed that military cooperation into a durable diplomatic framework that would shape Iberian and Atlantic affairs well into the modern period.

The longer-term consequences for Portuguese society and expansion were profound. The stabilization of the dynasty under the House of Avis, and the integration of energetic new noble families whose fortunes depended entirely on the new order, produced the institutional and social conditions from which the Portuguese Age of Discovery emerged within a generation. John I's sons — the princes who became known as the "Illustrious Generation" — would direct the first sustained European maritime expansion into Africa and the Atlantic, beginning with the capture of Ceuta in 1415^[1]. The interregnum had, paradoxically, cleared away a conservative aristocratic establishment whose ties to Castile and to the existing Iberian order might otherwise have reoriented Portugal landward rather than seaward.

What the war did not resolve was the underlying structural vulnerability of a small kingdom sharing the Iberian Peninsula with a larger and more populous neighbor. Castilian pressure on Portuguese independence would recur, culminating in the Iberian Union of 1580–1640, when dynastic accident again opened the succession and produced the outcome the defenders of 1385 had fought to prevent. The constitutional precedent of the Cortes electing a king likewise remained latent rather than generative: Portuguese monarchs of the Avis period governed with increasing personal authority, and the representative institutions that had legitimized John I's accession were never developed into a durable check on royal power.