Born into a large, prosperous Cuban farming family in Mayari, near Brian, Cuba, on August 13, 1926 or 1927, Castro attended Jesuit schools and later studied law in Havana. During his student years, he was a political activist. Upon receiving his degree in 1950, he established a private law practice and joined the reformist Cuban People's Party. In 1952 he planned to run for a parliamentary seat. However, Gen. Batista overthrew the government and canceled the election. Castro first challenged the Batista regime in court, but in 1953 organized an unsuccessful rebel force. Castro was arrested, tried and put in jail until 1955. He next went to Mexico to organize a new force, one that became known as the 26 of July Movement. In 1956 this group launched its attack, again meeting a bloody defeat. Castro and his followers next began a guerrilla war against the corrupt and by now very unpopular Batista regime. They quickly built a large following, also thanks to an effective propaganda campaign. On January 1, 1959, Castro triumphantly took power.

Having received widespread support from numerous Cubans because of his promises to restore the 1940 constitution, pursue a moderate program and respect civil liberties, Castro quickly disappointed many sympathizers with his radical policies after 1960. It turned out that the Cuban revolution involved a far-reaching transformation of Cuban society led by the state, which, for its part, was dominated by the communist party. The hostility of the United States could be taken for granted virtually from the start. Washington, under President Eisenhower, had already resolved to bring the Castro regime down, if necessary by violence. Disinclined to work with the Americans and in the face of U.S. hostility, Castro quickly intensified his cooperation with the Soviet Union. Cuban-U.S. relations deteriorated further when John Kennedy entered the White House, and they reached a crisis point with the CIA-sponsored Bay of Pigs invasion by Cuban exiles in April 1961. This event contributed heavily to the Soviet decision the following year to station nuclear missiles in Cuba. The Cuban Missile Crisis, during which Castro's influence was very limited, brought the world as close to the brink of nuclear war as it has ever been.

Castro remained fairly popular in Cuba. Many Cubans seemed to benefit from the redistribution, education and health policies of his regime. Those who opposed Castro often tried to flee the country for the United States before the regime could arrest them. Although Castro has never held an open election, many scholars argue that during the early years of his regime, he could have won one. Internationally, Cuba was solidly in the Soviet camp, also receiving considerable economic support from Moscow. During the 1970s Castro sent his own military intervention force to civil war-ridden Angola to support the communist side. The end of Soviet aid after 1991 has exposed the weaknesses of the Castro system, causing the regime to initiate some cautious reforms.