King of Castile and León (1379–1390) whose claim to the Portuguese throne through his wife triggered the 1383–1385 crisis and ended in catastrophic defeat at Aljubarrota.
John I was king of Castile and León from 1379 to 1390, the second monarch of the Trastámara dynasty founded by his father Henry II. His reign was dominated by two great dynastic contests: the Lancastrian claim to Castile pressed by John of Gaunt, and his own claim to the throne of Portugal.
Through his marriage to Beatrice, the only heir of Ferdinand I of Portugal, John claimed the Portuguese crown on Ferdinand's death in 1383, triggering the succession crisis known as the 1383–1385 Interregnum. His invasion ended in disaster at the Battle of Aljubarrota in 1385, which secured Portuguese independence. He died in a riding accident in 1390.
John was born in 1358, the son of Henry of Trastámara — the illegitimate half-brother of Peter the Cruel — and Juana Manuel. His father seized the Castilian throne as Henry II after killing Peter in 1369, founding the Trastámara dynasty, so John grew up as the heir of a usurping line whose legitimacy was still contested.
That contested inheritance shaped his reign from the start: the marriage of Peter the Cruel's daughter Constance to John of Gaunt gave the English prince a claim to Castile that John I would have to face down for much of his rule.
John I is remembered as a conscientious and reform-minded but unlucky king, more administrator than warrior. Castilian chronicle tradition, especially the account of Pero López de Ayala who served at his court, presents a ruler genuinely concerned with the government, law, and church of his kingdom, who summoned the Cortes frequently and took counsel seriously.
The same tradition, and the judgement of later historians, dwells on the misfortune and misjudgement that dogged him. The decision to press the Portuguese claim by force, and the reckless advance that ended at Aljubarrota, are remembered as the great errors of an otherwise careful reign. He appears as an earnest, dutiful ruler whose defining choices — the Portuguese war above all — brought catastrophe rather than the consolidation he sought.
John succeeded his father in 1379 and worked to stabilise the young Trastámara monarchy. He faced the most dangerous phase of the Lancastrian claim when John of Gaunt landed in Iberia to make good Constance's right to Castile; John I resolved the threat by diplomacy, arranging the marriage of his heir, the future Henry III, to Gaunt's daughter Catherine of Lancaster, which united the rival claims.
At home he was an active legislator who relied on the Cortes and on reform of the royal administration and the church. He also maintained Castile's French alliance, a legacy of his father's partnership with France against England during the Hundred Years' War.
In 1383 John married Beatrice, the only surviving legitimate child of Ferdinand I of Portugal, on terms meant to unite the two crowns. When Ferdinand died later that year leaving no son, John claimed the Portuguese throne in Beatrice's name, raising the prospect that Portugal would be absorbed into Castile.
Portuguese resistance rallied around John of Aviz, and John I invaded to enforce his claim. On 14 August 1385 his large army was destroyed at the Battle of Aljubarrota by the smaller Portuguese force under John of Aviz and the constable Nuno Álvares Pereira, fighting from a prepared defensive position with English longbowmen. The defeat shattered Castilian ambitions in Portugal and confirmed the House of Aviz on the Portuguese throne.
John I died on 9 October 1390 at Alcalá de Henares after a fall from his horse, aged only about thirty-two. His unexpected death left the throne to his young son Henry III, whose minority reopened the factional struggles of the Castilian nobility.