The Fernandine Wars: Ferdinand I’s Failed Castilian Ambitions (1369–1371)

Upon ascending the Portuguese throne in 1367, Ferdinand I of Portugal sought to expand his influence in Castile, taking advantage of the ongoing civil war between Henry II of Trastámara and the murdered Peter I of Castile.

Ferdinand’s Claim to the Castilian Throne (1369)

• In 1369, Peter I of Castile was assassinated by his half-brother, Henry of Trastámara, after a long civil war.
• As Peter’s closest legitimate male relative, Ferdinand claimed the Castilian throne, receiving support from several Castilian towns that remained loyal to Peter’s lineage.
• Henry II, now King of Castile, viewed Ferdinand’s claim as a direct threat and invaded Portugal in late 1369.

The First Fernandine War (1369–1371): A Humiliating Defeat

• The war quickly turned against Ferdinand, as Henry II’s forces—bolstered by French support—proved superior in strategy and manpower.
• Ferdinand failed to rally enough Castilian nobles to his cause and suffered several military defeats.
• The war ended in 1371 with the Treaty of Alcoutim, in which:
  • Ferdinand abandoned his claim to the Castilian throne.
  • Portugal was forced to pay Castile reparations.
  • Ferdinand agreed to marry Henry’s daughter, Leonor of Castile, to cement peace.

Political Fallout

• The treaty infuriated the Portuguese nobility, particularly when Ferdinand reneged on the arranged marriage and instead married Leonor Teles, a controversial and ambitious noblewoman.
• The failure of the war weakened Ferdinand’s credibility, yet he would continue to pursue Castilian ambitions, leading to two more Fernandine Wars (1372–1373, 1381–1382), further destabilizing Portugal.

This first disastrous war exposed Ferdinand’s military shortcomings and set Portugal on a path of continuous conflict with Castile, weakening the kingdom’s stability and paving the way for the crisis of 1383–1385, which would ultimately determine Portugal’s independence.