The trigger for the Cuban Missile Crisis occurred when reconnaissance photos taken by the United States of Cuba showed a significant Soviet build-up of strategic missiles. Cuba was 150 kilometres from the American mainland, and this was considered to be a considerable threat on behalf of the Soviets.
The Soviets were amassing these weapons in Cuba because they knew the Soviet Union was desperately behind the United States in the arms race. Soviet missiles were only powerful enough to be launched against Europe but U.S. missiles were capable of striking the entire Soviet Union. In late April 1962, Soviet Premier Nikita Khrushchev conceived the idea of placing intermediate-range missiles in Cuba. A deployment in Cuba would double the Soviet strategic arsenal and provide a real deterrent to a potential U.S. attack against the Soviet Union. Castro agreed to this plan, only because he was searching for a barrier to stop a US invasion.
When Kennedy ordered a quarantine put on Cuba and a naval blockade to be implemented, he announced the discovery of the missiles to the public. He also proclaimed that any nuclear missile launched from Cuba would be regarded as an attack on the United States by the Soviet Union and demanded that the Soviets remove all of their offensive weapons from Cuba.
During the public phase of the Crisis, tensions began to build on both sides. Both countries were now facing each other in a stand-off and neither would easily back down. As the world awaited the outcome, the 2 leaders, Kennedy and Khrushchev, exchanged letters. The Soviets demanded that the US not invade Cuba and also remove their missile sites from Turkey. The crisis peaked on October 27, when a U-2 was shot down over Cuba and another U-2 flight over Russia was almost intercepted. At the same time, Soviet merchant ships were nearing the quarantine zone. Kennedy responded by publicly accepting the first deal and sending Robert to the Soviet embassy to accept the second in private the small number of Jupiter missiles in Turkey would be removed. The Soviet ships turned back and on October 28 Khrushchev announced that he had ordered the removal of the Soviet missiles in Cuba.
Satisfied that the Soviets had removed the missiles, President Kennedy ordered an end to the quarantine of Cuba on November 20. As part of the deal he agreed not to invade Cuba to overthrow Fidel Castro and to withdraw the US tactical nuclear missiles from Turkey. The crisis had been safely averted.