Isabella I (1451-1504) was queen of Castile from 1474 to 1504. She and her husband, Ferdinand V, founded the modern Spanish state.

Born in Madrigal on April 22, 1451, Isabella was the daughter of John II of Castile by his second wife, Isabella of Portugal, and was the half sister of Henry IV, who succeeded to the Castilian throne in 1454. Henry had recognized Isabella as his heir over the claims of his daughter Juana, whose royal paternity was questioned by the King's opponents, but when Isabella married Ferdinand of Aragon in 1469, Henry conferred the succession on Juana.

When Henry died in 1474, Isabella immediately claimed the throne. In the ensuing civil war Juana was supported by a cross section of the great nobles as well as by the Portuguese king, Alfonso V. Alfonso's army was defeated at the battle of Toro in 1476, and he made peace with the Catholic Monarchs (as the pair...