From World Book Online for Students :

The Cuban missile crisis was a period in October 1962 when the United States and the Soviet Union teetered on the brink of war. The United States learned that the Soviet Union had secretly installed missiles in Cuba, about 90 miles (140 kilometers) from Florida. The missiles could have been used to launch nuclear attacks on American cities. The crisis was one of the most serious incidents of the Cold War. The Cold War was an intense U.S.-Soviet rivalry that had begun after World War II ended in 1945. Most experts believe that the missile crisis brought the United States and the Soviet Union close to nuclear war.

The Soviet Union had placed the missiles in Cuba earlier in 1962, after Cuban leaders became convinced that the United States was planning to attack Cuba. Cuba had undergone a communist revolution in 1959 in which Fidel Castro seized power. Castro's government had quickly become allied with the Soviet Union against the US. Under President Kennedy's leadership the US had supported an attempt to overthrow Castro's new government, but it failed (known as the Bay of Pigs invasion).

President John F. Kennedy of the United States learned of the missiles' presence on October 16. He demanded that the Soviet Union remove them. On October 22, he ordered a naval quarantine (blockade) of Cuba to stop further shipment of arms. In particular, the United States wanted to block the Soviets from sending nuclear material to arms the missiles.

At first, the United States expected to invade Cuba to destroy the missiles. At one point, an invasion was scheduled for October 29 or October 30. Nearly all of Kennedy's advisers agreed that a landing of U.S. forces in Cuba would probably mean war—most likely nuclear war—with the Soviet Union. Invasion, however, was not necessary. The Soviet Union offered to remove the missiles if the United States would promise not to invade Cuba. It later said that it would not remove the missiles unless the United States would dismantle its military bases in Turkey. Turkey was a U.S. ally that bordered the Soviet Union. Kennedy agreed publicly to dismantle all U.S. missile bases in Turkey. However, to complete the deal, Kennedy and Soviet leader Nikita S. Khrushchev also made a private agreement. In that deal, Khrushchev promised to remove all Soviet missiles in Cuba in exchange for Kennedy's promise that the United States would not invade the island. On October 28, the two leaders completed the agreement, ending the crisis.

The agreement between Kennedy and Khrushchev was kept secret because many Americans opposed such a deal. Almost all Americans thus thought that Kennedy had forced the Soviet Union to remove the missiles simply by threatening war. Kennedy came away from the incident looking like a bold, tough leader which greatly boosted his popularity. Some experts believe that, as a result, U.S. foreign policy used greater toughness and more threats of force after the crisis.