Nuclear war is the gravest existential threat facing the United States, and the world, at the moment: We are only one mistake away, and the margins are already thin and thinning, as Beltway hawks continue to press escalation over Ukraine. We shouldn’t believe “it can’t happen,” because it has almost happened several times.

There are three documented cases in which nuclear war nearly occurred: the Cuban Missile Crisis of 1962; the nuclear false alarm in the Soviet Union in 1983, when Stanislav Petrov defied protocol and refused to order a retaliatory strike in response to an erroneous radar reading; and 1995, when a Norwegian rocket launched to study the aurora borealis was mistakenly believed to be a US missile launch. In the last instance, the “suitcase” was brought to President Boris Yeltsin and preparations were made for a nuclear launch before the rocket veered away from Russian airspace. There may have been other instances that remain classified to prevent public fear—or demands for better safety measures or, say, monitored, phased mutual disarmament.

The margins of error in 1962 and 1983 were thin. In 1983, due to a series of seemingly unrelated events that Moscow perceived as causally linked, the Soviet military was already on heightened alert, a fact that renders Petrov’s restraint all the more impressive. We can’t know how the 1995 incident might have played out had US-Russian relations been as strained then as they are now.

The most studied of the three is, of course, the Cuban Missile Crisis. President John F. Kennedy, his brother Robert, and a few members of the administration shunned the considered-but-suicidal counsel of the Joint Chiefs and senior advisers, who proposed courses of action that would have, as was later learned, led to nuclear war.