IN MARCH 1970, President Richard Nixon took the stage at a brief but sweeping ceremony at the State Department where he signed the Treaty on the Nonproliferation of Nuclear Weapons. As international contracts go, the NPT, as it’s come to be known, has been hugely successful. Forty-seven years later, it binds 190 countries from spreading nuclear weapons.

“This is indeed a historic occasion,” Nixon said with Russian ambassador Anatoly Dobrynin looking on. “I only hope that those of us who were fortunate enough to be present will look back one day and see that this was the first milestone on a road which led to reducing the danger of nuclear war.”

Republican presidents have heeded Nixon’s call ever since. Gerald Ford, Ronald Reagan, and both Bushes all oversaw either the relative maintenance of the United States’s nuclear arsenal, or its significant reduction. After Reagan won reelection, he bluntly declared in his 1985 inaugural address, “We seek the total elimination one day of nuclear weapons from the face of the earth.” Then the former Hollywood actor put his money where his mouth was. Near the end of his second term, Reagan sat alongside Russian leader Mikhail Gorbachev in the East Room of the White House and signed the Intermediate-Range Nuclear Forces Treaty, which laid the foundation for several landmark nuclear arms-reductions agreements to follow.

But during the most recent presidential campaign, the Republican candidate Donald J. Trump zigzagged on the nuclear question, repeatedly sucker-punching Nixon’s hopes square in the face. Trump explicitly advocated nuclear proliferation. “North Korea has [nuclear weapons] […] I mean to me, that’s a big problem,” Trump told The New York Times in March. “Would I rather have North Korea have them with Japan sitting there having them also? You may very well be better off if that’s the case.” To Anderson Cooper the same month, he extended his argument to the Korean peninsula. “We’re better off, frankly, if South Korea is going to start to protect itself.”

With the zigs came the zags. “We have to be extremely vigilant and extremely careful when it comes to nuclear. Nuclear changes the whole ball game,” Trump said during a 2015 debate. “The biggest problem we have is nuclear proliferation and having some maniac, having some madman go out and get a nuclear weapon. That’s in my opinion, that is the single biggest problem that our country faces right now.”

Throwing the world for one more loop, on the Thursday before Christmas, hours after Russian President Vladimir Putin made a few bold nuclear statements of his own, Trump took to Twitter, and wrote, “The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes.”

Reconciling the opposing positions on nuclear proliferation from the man who is now going to be president is a challenge. So it’s useful to dive into two new substantial pieces of scholarship to remind ourselves of the current debates in the complicated conversation taking place today in nuclear policy. It’s an arena where the stakes are always high, and as Yale political scientists Alexandre Debs and Nuno P. Monteiro lay out in their new book Nuclear Politics, those stakes have pressed each of our most recent presidents, in the most serious way.

They write, “[Nuclear] proliferation concerns led President Bill Clinton to the brink of war with North Korea in 1994, were central to President George W. Bush’s case for invading Iraq, and pressed grave dilemmas on President Barack Obama concerning Iran.” In other words, combating nuclear proliferation has driven the last three US presidents to the brink of, or directly into, war.

So Debs and Monteiro set out to answer the question: why do some states acquire nuclear weapons and others don’t? Nuclear Politics is more than 500 pages of the most granular-level academic scholarship around, so short-handing their findings is inescapable. Much of their answer has to do with how much a state feels threatened by its adversaries, and how much it feels protected by its allies. They argue that isolated states facing big threats have been far more inclined to develop their own nuclear programs. Think Pakistan, which has long feared its rival India but lacked what it considered a reliable nuclear sponsor.

A quick aside: Walking through the several case studies in Nuclear Politics, it’s worth remembering that developing a nuclear weapons program is incredibly challenging. Building a program takes a significant amount of resources. And, even if you have both those bases covered, developing a nuclear weapons program takes many, many years — likely decades. So comments about South Korea or Japan just simply deciding to go out and get some nukes have to be taken with a serious dose of reality — any such undertaking would be gargantuan.

Whether you’re with the (comparatively large) crowd on the side of the looking-glass who believes that history and scholarship has taught us that proliferation, on its face, is a bad idea, or you’re someone who’s entertaining Trump’s suggestion that we may be “better off” if Seoul were to go nuclear, here’s another factor the Yale scholars have to offer. Once a state decides to develop a nuclear program, its adversaries will do everything in their power to deter them. Think about Iran over the last few years and the strange bedfellows (Israel and Saudi Arabia, for example) who were willing to band together to halt Tehran’s program.

Now think about South Korea and what the lengths North Korea would be willing to go to in order to halt a program to its south. Consider this scenario: Seoul starts a nuclear program, North Korea attacks to deter its development, and the United States is forced to intervene militarily on Seoul’s behalf.

The logic suggests Trump’s argument would be self-defeating for everyone involved. Force South Korea to develop its own nuclear weapon, only so the United States would have to launch a war to defend South Korea’s ability to continue building a bomb.

Then there’s the question of what having the bomb actually gets you. In Nuclear Weapons and Coercive Diplomacy, scholars Todd S. Sechser and Matthew Fuhrmann want to answer the question, “What has [the United States] been able to do with nuclear weapons that it could not have done without them?” Sechser and Fuhrmann challenge the notion that nuclear-armed countries are more threatening on the world stage, that nukes help win negotiations, take territory, and generally bend the will of other states to their own. “An emerging wisdom in international relations scholarship,” they write, “says that countries with large nuclear arsenals can bully other states into submission.” Then they slowly dismantle that notion with a deep academic dive into a range of international conflicts over the last seven decades and conclude that “nuclear weapons have far less utility for coercive diplomacy than many people believe.”

Why is that? Sechser and Fuhrmann make a point that Trump would be wise to review in light of his comments about possibly battling ISIS with nukes. “The military utility of nuclear weapons is limited,” they write, “and often redundant to the capabilities of conventional weapons.” In addition, the international norms that have emerged over the last seven decades of the nuclear age dictate that any use of nuclear weapons would spark a fierce international backlash, one so powerful from other states and the broader public, they suggest, that would threaten the “security, prosperity, and the political fortunes” of any country and its leaders who actually launched the nuclear attack.

Theory aside, there’s a singular feature of our command-and-control system. An aide-de-camp follows the president of the United States wherever he goes, every day of the year, carrying what’s known as the “atomic football.” It’s a black briefcase, and inside are the launch codes that allow one human being the power to unleash, in a matter of minutes, multiple intercontinental ballistic missiles equipped with nuclear warheads. He needs no signoff. He needs no approval from Congress, from the electorate, from anyone. There is no check on that authority, and it is the most devastating power held by any human being.

Last January, CBS’s John Dickerson asked Trump, “The United States has not used nuclear weapons since 1945. When should it?”

Trump replied: “Well, it is an absolute last stance. And, you know, I use the word unpredictable. You want to be unpredictable.”

Two months later, to Mark Halperin Trump said: “At a minimum, I want [ISIS] to think maybe we would use it, okay?” He went on: “The fact is, we need unpredictability.”

“Unpredictable” is an important explanation to consider when we’re talking about machinery capable of exterminating tens of millions of people in a few hours. Especially when most of the world’s nuclear powers — the United States, United Kingdom, France, Russia, Israel, India, Pakistan, China, and North Korea — have developed an incredibly sophisticated system and language for discussing nuclear matters, the main goal of which is to decrease the chance of any misunderstanding whatsoever, and consequently, lower the possibility of an accident that would trigger catastrophe.

“I will have a military that’s so strong and powerful, and so respected, we’re not gonna have to nuke anybody,” Trump said to GQ magazine in 2015. “It is highly, highly, highly, highly unlikely that I would ever be using them.”

The one word in our nuclear future to never forget: unpredictable.

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Andrew Bast is an Emmy-winning journalist for the CBS news magazine 60 Minutes. He’s finishing a screenplay about the birth of the atomic bomb.