The world came closest to nuclear war during the Cuban Missile Crisis. We find that there existed two paths by which nuclear war might have occurred. The first path involves unrestrained hard-liners. Nuclear weapons did not deter some actors from proposing escalatory actions, including the use of nuclear weapons. Luckily, both Kennedy and Khrushchev reined in their respective hard-liners. Along the second path, situations – not known at the time – could have led to an initial use of nuclear weapons, after which events might have spiraled out of control. The US, for example, did not know that the Soviets had placed tactical nuclear weapons in Cuba. If the US had tried to invade Cuba to topple Castro – as some people advocated – then the Soviets might have used the weapons. Ultimately, Kennedy successfully used a quarantine and threatened force to compel the Soviets to withdraw their missiles from Cuba. The threat of nuclear war lingered behind these actions. In the end, however, the crisis ended not because of nuclear deterrence but rather because both sides reached a mutually acceptable bargain. Kennedy promised not to invade Cuba and to remove US missiles from Turkey; Khrushchev, meanwhile, agreed to remove Soviet missiles from Cuba.

The 1948 Berlin Blockade did not escalate to war, largely because Truman and Stalin did not want another world war so soon. Atomic weapons played no obvious role. The fear of atomic weapons did not deter Stalin – who lacked such weapons – from initiating the crisis. They also played no role in ending the crisis, since Truman made no explicit threat to use them. Instead, the airlift defeated the blockade, and Stalin ended the crisis. In contrast, nuclear weapons played a clearer role in the 1958–1962 Berlin Crisis. Both sides now had a second-strike capability, but that deterred neither Khrushchev from initiating the crisis nor Kennedy from considering nuclear war. Nevertheless, the possibility of a nuclear war made each leader more prudent. As the crisis evolved, large numbers of East Germans started crossing into West Berlin. Ulbricht, the leader of East Germany, pleaded with Khrushchev to do something. Khrushchev permitted the construction of a wall (although Kennedy may have signaled his non-opposition). This ended the refugee flow and, therefore, the immediate crisis. The Berlin Crises intensified the Cold War. They led to the formation of NATO and the Warsaw Pact, as well as the division of Germany into two states.