On the evening of October 22, 1962, President Kennedy addressed the nation on television, revealing the presence of Soviet missiles in Cuba, evidence for which the US government had known about for some weeks. The next week would prove nervous indeed.
"Good evening, my fellow citizens," Kennedy began. "This government, as promised, has maintained the closest surveillance of the Soviet military buildup on the island of Cuba. Within the past week, unmistakable evidence has established the fact that a series of offensive missile sites is now in preparation on that imprisoned island. The purpose of these bases can be none other than to provide a nuclear strike capability against the Western Hemisphere...
"The size of this undertaking makes clear that it has been planned for some months. Yet, only last month, after I had made clear the distinction between any introduction of ground-to-ground missiles and the existence of defensive antiaircraft missiles, the Soviet government publicly stated on September 11 that, and I quote, 'the armaments and military equipment sent to Cuba are designed exclusively for defensive purposes,' that there is, and I quote the Soviet government, 'there is no need for the Soviet government to shift its weapons for a retaliatory blow to any other country, for instance Cuba,' and that, and I quote their government, 'the Soviet Union has so powerful rockets to carry these nuclear warheads that there is no need to search for sites for them beyond the boundaries of the Soviet Union.'
"That statement was false.
"Only last Thursday, as evidence of this rapid offensive buildup was already in my hand, Soviet Foreign Minister Gromyko told me in my office that he was instructed to make it clear once again, as he said his government had already done, that Soviet assistance to Cuba, and I quote, 'pursued solely the purpose of contributing to the defense capabilities of Cuba,' that, and I quote him, 'training by Soviet specialists of Cuban nationals in handling defensive armaments was by no means offensive, and if it were otherwise,' Mr. Gromyko went on, 'the Soviet government would never become involved in rendering such assistance.'
"That statement also was false...
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"The 1930s taught us a clear lesson: aggressive conduct, if allowed to go unchecked and unchallenged, ultimately leads to war. This nation is opposed to war. We are also true to our word. Our unswerving objective, therefore, must be to prevent the use of these missiles against this or any other country, and to secure their withdrawal or elimination from the Western Hemisphere....
"Acting, therefore, in the defense of our own security and of the entire Western Hemisphere, and under the authority entrusted to me by the Constitution as endorsed by the Resolution of the Congress, I have directed that the following initial steps be taken immediately:
"...To halt this offensive buildup a strict quarantine on all offensive military equipment under shipment to Cuba is being initiated. All ships of any kind bound for Cuba from whatever nation or port will, if found to contain cargoes of offensive weapons, be turned back. This quarantine will be extended, if needed, to other types of cargo and carriers. We are not at this time, however, denying the necessities of life as the Soviets attempted to do in their Berlin blockade of 1948...
"It shall be the policy of this Nation to regard any nuclear missile launched from Cuba against any nation in the Western Hemisphere as an attack by the Soviet Union on the United States, requiring a full retaliatory response upon the Soviet Union..."
The entire speech is here, as a transcript, audio file and video clip.