Isabella and her husband Ferdinand were married secretly

When Isabella was just six years old, the marriage between her and Aragon was planned. But their union happened a long time afterward. The understanding between Henry and Isabella suggested that Isabella would not be forced into a marriage. She was not meant to be compelled to wed someone Henry didn't like.

In actuality, Henry broke his word. In the final years of his rule, he was quickly losing influence. To strengthen his position, he required fresh allies. He made numerous attempts to plan his sister's wedding. Each country put out a marriage candidate: Portugal, Aragon, and France. It appears that Henry intended Afonso V, king of Portugal, and his half-sister to wed. The potential political advantages of their marriage were his first focus.

Henry ignored Isabella's reassurances that he had kept his pledge to her. Finally, she decided to secretly wed Ferdinand, the man she had known since she was a young child. They had several issues because they were second cousins. She had to get the Pope to give him a dispensation before she could legally wed Ferdinand. The couple later wed in secrecy in 1469, uniting Eastern and Western Spain and making it a strong force in Europe.