The Cuban Missile Crisis of October 1962 is considered the closest the world has ever come to a full-scale nuclear war.
At the time of the missile crisis, Cold War tensions between the United States and the Soviet Union were extremely high. Soviet leader Nikita Khrushchev was prone to bluster and threatening rhetoric and believed that he could intimidate the young, new American president, John F. Kennedy. In August 1961, Khrushchev authorized the construction of a barrier separating the eastern and western halves of the city of Berlin, effectively closing off the U.S.-protected enclave from the surrounding world. The 1959 Cuban revolution 90 miles off the coast of the United States had already inflamed tensions between the two superpowers over the direction in which Fidel Castro was taking his country, and the badly bungled U.S.-backed invasion at the Bay of Pigs was seen in the Kremlin as having seriously weakened President Kennedy.
Khrushchev's decision to place nuclear weapons in Cuba has been interpreted as intended to both prevent further U.S. military actions against the new Soviet ally in Havana and to radically shift the overall balance of power between the U.S. and the U.S.S.R. By placing nuclear-armed missiles in Cuba, the Soviet Union could now destroy targets in the United States in a matter of seconds -- just as the Soviets calculated the United States could do to it with U.S. nuclear weapons based in western Europe.
The ensuing crisis brought U.S. and Soviet naval forces into close contact in a situation where any mistake by a commanding or subordinate officer on a ship or submarine belonging to either side could conceivably have triggered a full-scale war. In addition, U.S. reconnaissance flights over Cuba, intended to locate and track the Soviet nuclear weapons, were a source of great irritation to Castro. When one of the reconnaissance flights was shot down by a surface-to-air missile, killing the American pilot, an already extremely tense situation grew more so.
How much the people in the Soviet Union knew about the missile crisis is uncertain, given total control over sources of information by the government there at that time. In the United States, however, the entire country held its collective breath until the crisis ended. The fear of nuclear war was real, and the public truly believed it had come too close to annihilation.