Some fun facts about the
Cuban Missile Crisis...
One of Kennedy's primary advantages over the Soviet Union was that he was in possession of a series of reports from a spy named Col. Oleg Penkovskiy, who had 'defected' to the CIA and remained in Moscow to provide information. Penkovskiy's reports had, for several months, indicated unequivocally that the Soviets were, in fact, nowhere near as advanced in their missile technology and deployment as American analysts and policy makers had thought them to be.
One of the major motivations Khruschev had for the risky deployment was that he was being threatened by internal enemies for having failed to achieve strategic parity with or dominance over the United States and NATO. While this alone would not necessarily have doomed him, he had been for some time bolstering the Soviet position through brash assertions of nonexistent forces.
While it is true that the U.S. had already figured this out to some degree via the pre-Francis Gary Powers debacle through U-2 overflight intelligence, it was at the time one of those situations where 'we know, they know we know, we know they know we know, but the rest of the world doesn't really know.' The exposure of the Soviet Union's weak hand, as evidenced in its withdrawal from (as has been noted above) no more than an achievement of parity, nailed some of the final brads into Nikita's political coffin.
Another fun fact: when told on Oct. 19th or 20th (pre-blockade announcement) to deploy his naval group immediately, a U.S. navy commander asked his superior "Against what?" He was told that that was still on a need-to-know basis. Frustrated, he asked which way he was to go once clearing his East Coast anchorage and harbor point; the answer was "Turn right."
(Note: The following is opinion!)
Still, it was right and proper for Kennedy to oppose the Soviet maneuver. To fail to do so would have indicated either that the U.S. hadn't the political will to back its claims, and/or that the Monroe Doctrine (a dearly-held bit of U.S. rhetoric) had no weight. Given the problems already existant in the U.S. from COMINTERN and the various propaganda and intelligence arms of the Communist Party, such an indication could have caused internal chaos as the subversive elements already active would have regarded this as a 'green light.'