The prolif domino theory is correct and spread ensures miscalculation that heightens the risk of war

But countries do not obtain the bomb in a vacuum. The nuclear posture and strategic decisions of nuclear-armed nations have a significant, often immediate, impact on the nuclear acquisition decisions of other nations. More simply: If your adversary or your neighbor goes nuclear, you will, too.

There is nothing automatic about the nuclear domino theory, and it has been successfully countered in some regions, but the theory is generally correct. The Soviet Union got the bomb because, as Stalin told his scientists after Hiroshima, “The balance has been broken. Build the bomb. It will remove the great danger from us.” Britain and France got the bomb because the Soviets (and the U.S.) had it. China did the same, then India got the bomb because China did; Pakistan because India did.

Nuclear competition in Asia would not end if South Korea decided to build a nuclear arsenal. Others in the region would likely follow suit. Japan, Taiwan, perhaps Vietnam. Similarly, a Saudi bomb would likely beget an Iranian bomb, a Turkish bomb and even an Egyptian bomb. Far from making the region — and the United States — safer, these arms races would blanket the globe with nuclear tripwires, each primed to unleash unprecedented destruction at the slightest twitch.

Where you stand determines what you see. Kennedy and the other presidents stood atop the chain of command, and their own experiences with that awful responsibility (particularly with the near-miss of the Cuban Missile Crisis) colored how they saw nuclear politics. They recognized the limitations of theory in a world characterized by imperfect information and the frictions of human interaction. They understood what the nuclear theorists could not — that more countries having nuclear weapons would only increase the risk of their use, not lessen it.

Three months before the Cuban Crisis, Kennedy’s Secretary of Defense, Robert McNamara, gave a speech in Ann Arbor, Michigan where he laid out this danger. “The mere fact that no nation could rationally take steps leading to nuclear war does not guarantee that a nuclear war cannot take place,” he said. “Not only do nations sometimes act in ways that are hard to explain on a rational basis, but even when acting in a ‘rational’ way they sometimes, indeed disturbingly often, act on the basis of misunderstandings of the true facts of a situation. They misjudge the way others will react, and the way others will interpret what they are doing.”

Any attempt to rationalize nuclear relationships — treating adversaries like two sides of a balanced equation — removes the human factor: the tendency towards irrationality and error. In a world with just a handful of nuclear states, that factor has already nearly led to apocalypse. In a world with a dozen more, those risks would go up exponentially.

It does not have to be this way. For over 50 years, since the signing of the Nuclear Non-Proliferation Treaty, successful diplomacy, security assurances, and global norms have largely kept nuclear proliferation at bay. The nightmare scenario of dozens of nuclear states has so far been averted, in no small part through the conscious and continual effort of American presidential administrations of both parties. Yes, there will always be those who advocate for more nuclear weapons in more hands. But the forces of restraint, and with it, survival, have prevailed and can continue to prevail if U.S. policy leads the way.

That is the beauty of nuclear policy: it leaves a track record. We can look back over the 75 years of the nuclear age and see what has worked to make the world safer and what has made it more dangerous.

When we let the nuclear virus spread unchecked, without any mitigation or suppression efforts, the contagion went from one nuclear nation in 1945 to five by 1964. From two bombs to over 34,000. But when we applied controls like test ban treaties, strategic arms limitations, nuclear non-proliferation accords and, most importantly, reduction treaties, we were able to flatten the nuclear curve, to limit the spread to just the nine nations today and to slash arsenals by over 85 percent from their peak of some 66,000 weapons in the mid-1980s to just under 13,400 today.