Henry of Trastamara, who would become Henry II of Castile, was born on 13 January 1334 as the fourth of ten illegitimate children born to King Alfonso XI of Castile and his mistress Eleanor de Guzman. Eleanor was herself of distinguished lineage, being a great-granddaughter of King Alfonso IX of Leon. Henry was born a twin alongside Fadrique Alfonso, Lord of Haro, and was the first of Alfonso XI's illegitimate sons to survive to adulthood. At birth, he was adopted by Rodrigo Alvarez de las Asturias, a powerful noble whose death the following year left Henry as heir to the lordship of Norena. His father would subsequently make him Count of Trastamara and lord of Lemos and Sarria in Galicia, along with the towns of Cabrera and Ribera, a substantial territorial base in the northwest of the Iberian Peninsula that would eventually lend its name to an entirely new royal dynasty.
The death of Alfonso XI in March 1350 from fever during the siege of Gibraltar altered everything. The legitimate heir Peter ascended the throne, and his mother Queen Maria of Portugal, who had long resented Eleanor de Guzman's privileged position at court, moved quickly. Eleanor was arrested and eventually executed at Talavera de la Reina. Henry and his brothers scattered, fearful of what the new king might do to them. Although Eleanor and her sons had initially reached an accommodation with Peter, the underlying tensions could not be contained. Henry and his brothers Fadrique, Tello, and Sancho staged a succession of rebellions against Peter, whose reign earned him the epithet Peter the Cruel, though his supporters preferred to call him Peter the Just.
To strengthen his position and acquire reliable allies, Henry made a strategically significant marriage to Juana Manuel, daughter of Juan Manuel, Prince of Villena, the most powerful nobleman of the realm. In 1351, Henry fled to Portugal after one of the failed revolts. He was pardoned by Peter and returned to Castile, then led another revolt in Asturias in 1352. A cycle of reconciliation and renewed rebellion followed, eventually ending with Henry's flight to France, where he entered the service of King John II of France. Around this time he also spent a period fighting in the army of King Peter IV of Aragon during that kingdom's war against Castile, beginning in 1358. He was defeated and captured at Najera in 1360, held prisoner until his liberation with the assistance of Juan Ramirez de Arellano, among others, after which he went into exile in France again.
The decisive phase of Henry's contest for the Castilian throne began when Peter IV of Aragon launched another attack on Castile and invited Henry to participate on the condition that Aragon would actively support the overthrow of Peter I. This alignment became the Castilian Civil War. The combined forces of Henry's Castilian supporters, the Aragonese army, and a company of French mercenaries led by the renowned commander Bertrand du Guesclin invaded Castile. Peter I, expelled from his own kingdom, took refuge in Guyenne in the English dominions in southwestern France, where Edward, the Black Prince, held court. Edward agreed to restore Peter to his throne, and despite his army suffering catastrophically from dysentery during the Iberian campaign, he led his forces to victory at the Battle of Najera on 3 April 1367, defeating Henry's coalition and temporarily restoring Peter I.
Henry was not finished. He regrouped and eventually succeeded in cornering his half-brother. The final confrontation took place in 1369 at Montiel, where Peter was captured. Henry killed him personally, or at the very minimum was present when he was killed, an act of fratricidal violence that gave him the permanent epithet el Fratricida, the Fratricidal. He had already been proclaimed king at the Abbey of Santa Maria la Real de Las Huelgas in Burgos in 1366, but Montiel made his claim definitive. The price of his victory was the enormous debt he owed to his allies. He was compelled to reward them with titles, lands, and wealth on a scale that reshaped the Castilian nobility and earned him a second nickname, el de las mercedes, he of the mercies, a reference to the liberal grants of favor he was obliged to distribute.
As King of Castile and Leon, Henry II found himself almost immediately entangled in the broader European conflicts of the era. He became involved in the Fernandine Wars against Portugal and was drawn into the Hundred Years' War between France and England, continuing the alliance with France that had made his victory possible. His reign as the first king from the House of Trastamara represented a dynastic revolution, the illegitimate branch of the Castilian royal house supplanting the legitimate line through violence and civil conflict. Henry II of Castile died on 29 May 1379, leaving a kingdom that he had won through blood and held through patronage, and a dynasty that would rule Castile for more than a century after his death.
