JOHN (JUAN) I. 1358-1390), king of Castile and Leon, born in August 135S, was the son of Henry ("El Bastardo"), whom he succeeded in 1379. At his accession the Lancasterian claims to the throne of Castile were renewed, and gained the support of Portugal ; the result was a war with the latter power, which ended in a marriage (1382) between John and the Portuguese infanta. The peace thus ratified did not subsist long, for, on the death of Ferdinand of Portugal in the following year without male issue, John sought to establish a claim to the succession on behalf of his wife, and crossing the frontier penetrated as far as to Lisbon, to which he began to lay siege while John, the grand-master of Aviz, was being proclaimed king. Compelled by pestilence and other unfavourable circumstances to withdraw, he encountered the Portuguese in the neighbourhood of Aljubarrota in August of 1335 ; the disastrous defeat he there sustained was fullowed by a descent of John of Gaunt, duke of Lancaster (July 1386), which led to the conclusion of the peace of Troncoso (1387), in virtue of which the constantly recurring disputes about the crown were settled by the marriage of the crown prince Henry to Catherine, the representative of the Lancasterian claims. The last four years of the reign of John were marked by important legislative reforms in the town brotherhoods (kermandades), in the army, and in the system of taxation. In 1390 he was killed by a fall from his horse, and was succeeded by his son Henry III.