Mr. Castro made the boy another symbol of American oppression, which diverted attention from the deteriorating conditions in Cuba. After several months, American agents seized the boy from his Miami relatives and returned him to his father in Cuba, where he was greeted by Mr. Castro.

That episode carried great significance for Mr. Castro in the way it echoed one in his personal life.

Mr. Castro and his wife, Mirta Díaz-Balart, divorced in 1955, six years after the birth of their son, Fidelito.

In 1956, when Mr. Castro and Ms. Díaz-Balart were both in Mexico, Mr. Castro arranged to have the boy visit him before embarking on what he said would be a dangerous voyage, which turned out to be his invasion of Cuba. He promised to bring the boy back in two weeks, but it was a trick. At the end of that period, Mr. Castro placed Fidelito in the custody of a friend in Mexico City. He then sailed for Cuba with his fellow rebels on the yacht Granma.

The boy’s mother, with the help of her family and the Cuban Embassy in Mexico City, found a team of professional kidnappers, who ambushed the boy and his guardians in a park and carried him off. Ms. Díaz-Balart took Fidelito to New York and enrolled him in a local school for a year. But after Mr. Castro entered Havana and grabbed control of the government, he persuaded his former wife to send the boy back. The younger Mr. Castro lived in Cuba until, years later, he was sent to the Soviet Union to study. He became a physicist, married a Russian woman and eventually returned to Cuba, where he was named head of Cuba’s nuclear power program.

Details of Mr. Castro’s personal life were always murky. He had no formal home but lived in many different houses and estates in and around Havana. He had relationships with several women, and only in his later years was he willing to acknowledge that he had a relationship of more than 40 years with Dalia Soto del Valle, who had rarely been seen in public. (Whether they were legally married was not clear.)

The two had five sons — Alexis, Alexander, Alejandro, Antonio and Ángel — all of whom live in Cuba. Mr. Castro also has a daughter, Alina, a radio host in Miami, who bitterly attacked her father on the air for years.