Why Henry IV chose his sister as heir over his daughter

Henry IV did not choose Isabella willingly: the nobility imposed his sister on him

Henry IV of Castile did not choose his sister Isabella as heir to the throne. She was imposed on him at swordpoint and under the shadow of an accusation of bastardy against his own daughter, Joanna. The decision, far from being a gesture of fraternal trust, was the result of a noble revolt that questioned the princess's legitimacy and brought Castile to the brink of civil war.

The Castilian nobility, led by the great lineages, fueled for years the rumor that Joanna was not the king's daughter, but rather of Beltrán de la Cueva—hence the nickname "la Beltraneja". With that excuse, they proclaimed Alfonso, Henry's half-brother and Isabella's brother, as king. Henry responded with arms, but his victory was partial and his clemency toward the defeated, counterproductive.

When Alfonso died as a teenager—at just 14 or 15 years old, according to chronicles—the rebels did not relent. They threatened to place Isabella on the throne. Henry, besieged and without firm support, gave in in 1468 with the pact of the Toros de Guisando: he would continue reigning until his death, but Isabella would be the heir. In exchange, the infanta Joanna was promised to the King of Portugal, Afonso V, who claimed her as his vvife—he at 43, she at 13—to unite the crowns.

The maneuver did not end the conflict. Henry later tried to revoke the pact and restore Joanna, but he died in 1474 without succeeding. The War of the Castilian Succession then broke out, pitting the supporters of la Beltraneja—backed by Portugal—against those of Isabella. The outcome is well known, but the controversy over Joanna's legitimacy never fully dissipated.

What weighed more in Henry's decision: military pressure or the conviction that his daughter was not his? Historians still disagree. What is clear is that the choice of Isabella as heir was not an act of will, but a forced surrender. And that the crown of Castile changed hands because of a rumor, a war, and a child marriage.