Further you seem to forget that Kennedy had already announced to the entire world that any nuclear attack by the Soviet Union from Cuba on the United States or any US ally or any country in this hemisphere be considered an act of war y the United States requiring a full nuclear response by the United States upon the Soviet Union. What the hell do you think "full nuclear response" means? Do you actually think Kennedy could have backed down after that statement?
It's a bit complicated. It's true that the Soviets were ramping up their missiles and didn't have that many. While JFK had run on the myth of a 'missile gap' that the Soviets were ahead, when he took office he was informed that our intelligence said they actually had only four missiles. I'm not sure how many they had 2.5 years later. So perhaps they couldn't have completely destroyed the US, only several major cities. Not exactly a great outcome. I also don't have info on the fallout effects from a war at that time.
Had we invaded as Kennedy's government wanted, and the Soviets used their tactical nukes, it seems clear Kennedy would have returned nukes to at least Cuba. Here's where a little more background helps.
Eisenhower had removed conventional forces and made all-out nuclear war our only option to respond to a variety of situations, making the world a very dangerous place. Kennedy saw that hair-trigger for nuclear was as the most dangerous thing in the world and made it his top priority to change.
When he took office, he ordered McNamara to review our nuclear war plans, which were designed and held under the insane Curtis LeMay. Turns out, at the time, the Pentagon felt that the nation's nuclear war plans were 'theirs' and no civilian had ever seen them, and they were not interested in letting their boss, the Secretary of Defense, see them, and they told McNamara no.
McNamara had to go to JFK and have him issue an order to the Pentagon to let him see the plans. When he did, he saw that the only plan was an 'all out' attack destroying not only every decent size city in the Soviet Union, but just because, in China as well.
But there's a little more to the story. Turns out, JFK and McNamara had a real problem with the idea of nuclear war. They made a secret pact that if the a report came in of Soviet missiles launched at the US, JFK would not automatically launch missiles in response. Their thinking was that at that point, deterrence would have failed, and killing that many more people in response would not necessarily be the right thing to do, and that they knew the detection systems were imperfect and they wouldn't trust them; of course, since deterrence was so important, they kept this agreement secret to the two of them. Privately, JFK called himself "almost a peace at any price president" - but as the country demanded a cold warrior at the height of the cold war, he knew to portray that.
So having said all that, it's not quite clear how much the nuclear war would have escalated in the Cuban Missile Crisis, but it's though it would have been a global exchange - and there's yet another issue to account for. JFK and Khrushchev both had large "hawks" in their military they felt were threatening their control over their countries they couldn't necessarily control. JFK was shown a book, "Seven Days in May", with a story about the military taking control, and told his friends he felt it was possible if there was one more Bay of Pigs type incident where he held the military back; he asked for it to be made into a movie as a warning to the country, and allowed filming in the White House.
He and Khrushchev discussed this danger of not being able to control their militaries (both sides had lower level officers that nearly started a war) and rather became allies against their own militaries. Khrushchev used a metaphor of 'pulling a knot tighter' which they'd be unable to loosen. For that matter, JFK was using 'backchannel' communications to talk to Khrushchev, where Robert Kennedy would meet with a trusted Soviet contact, to keep their communications even from his own government - and the final resolution to the crisis involved a 'secret deal' that would have been politically disastrous for Kennedy if exposed at the time. McNamara was really Kennedy's only trusted military ally at the time, though he tried to improve things a little by bringing in a new Joint Chief of Staffs Chairman, Maxwell Taylor, who had protested Eisenhower's nuclear hair-trigger.
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