Even if the submarine had fired a conventional torpedo, that probably would have escalated very quickly.
In "The Fog Of War", McNamara, who was Secretary of Defense at the time tells a story of a meeting between him, Castro, and the man who was commander of the Warsaw treaty troops at the time (I forgot his name, though, sorry). The US apparently knew there were missiles on Cuba, but thought there were no warheads, yet. At this meeting (~1992-ish, I think), the Russian tells McNamara there were indeed enough warheads on Cuba at the time to destroy all major cities on the east coast of the US. So McNamara asks Castro if he knew of that and if he would have advised the Soviets to use the nuclear weapons on Cuba to retaliate against a US attack. Castro, apparently knew of the warheads and had urged the Soviets to use them in case of an attack, knowing full well that Cuba would have been destroyed completely. (At least that is how I remember McNamara telling the story, and assuming, of course, McNamara remembers the incident correctly and is telling the story accurately.)
Even if the submarine had fired a conventional torpedo, that probably would have escalated very quickly.
In "The Fog Of War", McNamara, who was Secretary of Defense at the time tells a story of a meeting between him, Castro, and the man who was commander of the Warsaw treaty troops at the time (I forgot his name, though, sorry). The US apparently knew there were missiles on Cuba, but thought there were no warheads, yet. At this meeting (~1992-ish, I think), the Russian tells McNamara there were indeed enough warheads on Cuba at the time to destroy all major cities on the east coast of the US. So McNamara asks Castro if he knew of that and if he would have advised the Soviets to use the nuclear weapons on Cuba to retaliate against a US attack. Castro, apparently knew of the warheads and had urged the Soviets to use them in case of an attack, knowing full well that Cuba would have been destroyed completely. (At least that is how I remember McNamara telling the story, and assuming, of course, McNamara remembers the incident correctly and is telling the story accurately.)