General Notes

John I (of Castile and León), in Spanish, Juan I (1358-1390), king of Castile
and León from 1379 to 1390. He was the son of King Henry II, who founded the
house of Trastamara. John attacked Portugal to defeat the alliance created by
King Ferdinand I of Portugal and John of Gaunt, duke of Lancaster, who had a
claim to the throne of Castile through his wife. Ferdinand submitted in 1382
and made peace, giving his daughter to John I in marriage. On Ferdinand's
death in 1383, John I again made war on Portugal, but was defeated at
Aljubarrota in 1385 by King John I of Portugal. From 1386 to 1387 John I
repulsed John of Gaunt's invasion of Spain and settled their differences by a
treaty and by arranging the marriage of one of John of Gaunt's daughters to
his son Henry, later King Henry III of Castile and León.