Although the Cuban Missile Crisis was touted as “America standing firm against Soviet expansion”, it was actually the other way around.
The crisis started because of the threat of the U.S. invading Cuba plus the fact that the US had just placed nuclear missiles in northern Turkey.
The Russian convoys to Cuba were not halted because of the “firm stand” by Kennedy but by Kennedy’s agreement to remove the missiles in Turkey and a promise not to invade Cuba.
MAD (Mutually Assured Destruction) is one of the factors that saved us from global catastrophe. Those in power on both side realized that it was a lose-lose situation, and their power would be lost by starting a nuclear war. The most dangerous times were early on before the build up of weapons when either side could have believed a pre-emptive strike would be successful. By the time the nuclear arsenals hit the worldwide destruction level it was unlikely that they would be used other than to counter a conventional invasion. Kennedy brought us close to the brink with the Bay of Pigs, and when China began to build a nuclear arsenal they appeared at times to be close to engagement with the Soviets, something largely not seen by the west who operated on the simplistic mindset that the China and the USSR were staunch allies.
Reagan looked like a monkey playing with a wrist watch when it came to nuclear arms, at least as far as his publicly stated policies. They sometimes sounded like he wanted to upset the balance of power. However, I think the Soviets were easily convinced that he was playing internal US political games, something they understood well, and actually the results of negotiations was to maintain parity.
Not the Soviets and not MAD, but you could certainly look to the Formosa Straits crises in the 1950’s. Sec’y of State John Foster Dulles believed in “brinksmanship,” but the Joint Chiefs had approved the use of nuclear weapons against Communist China. Eisenhower overruled them.
Don’t forget that the Soviet/Chinese MO was to actively ferment Communist revolutions anywhere they could, and their propaganda was very belligerent toward the West and capitalism just for existing, rather than for anything in particular that the West planned to do to the communist world.
In other words, they seemed very much like a threat, because they did unthinkable things internally, their propaganda was threatening to the West, and they actively supported people trying to overthrow nations friendly to the West.
From what I can recall of Kennan’s writings is that he said the Soviet Union would always back down when faced with the prospect of actually going to war with the United States. They’ll bluster and threaten but they don’t actually have it in them to pull the trigger.