Castilian Civil War

The immediate occasion for the civil war was the death of Nájera-era Castile's reigning monarch, Alfonso XI, from plague at the siege of Gibraltar in March 1350^[1]. He left behind a legitimate heir, Peter, who succeeded him as king, and a cluster of illegitimate sons born of his long-standing relationship with Eleanor de Guzmán. The eldest of these, Henry of Trastámara, would become Peter's principal challenger and, in time, his nemesis. The succession was not inherently contested on constitutional grounds — Peter's legitimacy was unimpeachable — but the personal and political circumstances surrounding the new reign made violent confrontation almost inevitable.

The structural cause was the consolidation of a powerful noble faction around the Trastámaran line. Alfonso XI had lavished titles, lands, and revenues on his illegitimate sons, creating a magnate interest with both the resources and the dynastic pretext to resist centralized royal authority. Peter's attempts to curtail noble power and reassert the crown's prerogatives alienated precisely those great houses whose cooperation a new king needed most^[1]. His treatment of Eleanor de Guzmán — who was arrested and subsequently killed in the early years of his reign — transformed a political rivalry into a blood feud. Henry of Trastámara emerged as the focal point for every Castilian magnate who had reason to fear or resent Peter's rule.

The conflict acquired an ideological edge through the framing of Peter's kingship by his opponents. His enemies, and later much of subsequent historiography, characterized him as "the Cruel," a ruler whose elimination of rivals — noble, clerical, and familial — exceeded what even a tough-minded medieval court would sanction^[1]. Whether this characterization reflects genuine tyranny or Trastámaran propaganda is genuinely contested, but the political effect was real: it provided moral cover for rebellion and made it easier for foreign powers to present intervention as a form of justice rather than mere opportunism.

That foreign dimension proved decisive in shaping the war's scale and duration. The Castilian succession struggle was absorbed into the broader framework of the Hundred Years' War, in which France and England competed for strategic advantage across Western Europe^[1]. Castile's Atlantic and Mediterranean naval capacity made it a prize worth backing. France and its Valois client network supported Henry of Trastámara, while England under Edward III and later the Black Prince backed Peter, who offered commercial and military concessions in return. French routier companies under Bertrand du Guesclin provided Henry with seasoned infantry that Castilian noble levies alone could not have matched. The intervention of these external powers extended a dynastic quarrel into an eighteen-year war and embedded its outcome within the larger contest for European supremacy.

The civil war ended in April 1369 when Henry of Trastámara — having been defeated at Nájera two years earlier by the Black Prince's Anglo-Gascon expedition — cornered King Peter I at Montiel and killed him in single combat, or at least in close personal confrontation following the castle's siege. No formal peace treaty was required: the dynastic question was settled by Peter's death and the extinction of his direct line. Henry II was recognized as king of Castile, inaugurating the Trastámara dynasty that would govern Castile, and eventually a united Spain, for well over a century ^[1].

The political consequences within Castile were substantial. Henry II had purchased his victory through extensive grants of land, revenues, and privileges — the so-called *mercedes enriqueñas* — to the Castilian nobility who backed his cause ^[1]. These concessions decisively strengthened the high nobility at the expense of royal fiscal and administrative authority, establishing a pattern of aristocratic power that successive Trastámara monarchs would spend generations attempting to claw back. The Jewish communities of Castile, many of whom had served Peter I in financial and administrative roles, were targeted in the aftermath of his defeat; anti-Jewish violence intensified across the following decades, foreshadowing the pogroms of 1391 that devastated Iberian Jewry ^[1].

The war's resolution had immediate and lasting consequences for the broader contest of the Hundred Years' War. Castile under the Trastámaras aligned firmly with France, providing the French crown with access to Castilian naval power that had been denied to it under Peter I's pro-English orientation. The Castilian fleet's capacity in Atlantic and Channel waters proved strategically significant: Castilian warships raided English coastal towns and disrupted English supply lines, materially worsening England's strategic position in the decades following 1369 ^[1]. The Black Prince's costly intervention at Nájera — which restored Peter temporarily but yielded no durable English advantage — accelerated the financial and political deterioration that marked his final years and complicated Edward III's war effort.

Geopolitically, the Trastámara victory reinforced the Franco-Castilian alliance that shaped Iberian and Atlantic affairs for the remainder of the fourteenth century and into the fifteenth. Portugal, which had broadly supported Peter I, found itself increasingly exposed by Castile's new alignment, contributing to the tensions that produced further conflict on the peninsula — most consequentially, the crisis of 1383–1385 in which the Portuguese successfully defended their independence at Aljubarrota. Aragonese interests were likewise recalibrated: the Trastámara dynasty eventually extended its reach to Aragon following the Compromise of Caspe in 1412, drawing the separate Iberian crowns into the dynastic consolidation that culminated under Ferdinand and Isabella at the end of the century ^[1].

What the civil war did not resolve was the underlying volatility of Castilian royal authority. The nobility's augmented power, the weakened crown revenues, and the social dislocations attendant on nearly two decades of intermittent warfare left Castile structurally fragile. Royal minorities, noble faction, and periodic civil strife continued to afflict the kingdom under Henry II's successors through much of the fifteenth century, until Ferdinand and Isabella undertook the systematic reassertion of royal power that is properly the next chapter of Castilian political history ^[1].