John II of Castile defeats the Infantes of Aragon at the First Battle of Olmedo.

John II of Castile was King of Castile and León from 1406 to 1454. He succeeded his older sister, Maria of Castile, Queen of Aragon, as Prince of Asturias in 1405.

The Infantes of Aragon is an appellation commonly used by Spanish historians to refer to a group of 15th-century infantes (princes) of the House of Trastámara, specifically the sons of King Ferdinand I of Aragon and his wife Eleanor of Alburquerque:

• Infante Alfonso (1396–1458) – became Alfonso V of Aragon, also king of Sicily and Naples (f.1442)
• Infanta Maria (1396–1445) – Maria of Aragon, first wife of John II of Castile (m.1420)
• Infante Juan (1398–1479) – King of Navarre (f.1425), later King John II of Aragon (f.1458).
• Infante Enrique (1400–1445) – Henry of Aragon, Duke of Villena, Count of Albuquerque, Count of Empúries and Grand Master of the Order of Santiago (f.1409)
• Infanta Leonor (1402–1445) – Eleanor of Aragon, consort of Edward I of Portugal (m.1428)
• Infante Pedro (1406–1438) – Peter of Aragon, Count of Alburquerque and Duke of Noto
• Infante Sancho (1410–1416) – infant master of the Order of Alcántara, died in childhood

The First Battle of Olmedo, between Castilian forces and those of Navarre and Aragon, took place on 19 May 1445 outside Olmedo in Castile.