A Trump presidency may seem increasingly improbable, but it also still remains conceivable. This means, among other things, that a President Trump armed with America’s nuclear codes is still more-or-less plausible. It therefore follows that if this particular conjunction should come to pass on Inauguration Day in January 2017, a number of genuinely urgent issues concerning presidential war authority would require our rivetingly prompt attention.

In essence, we Americans would finally have to really ask ourselves: “What if an American president becomes irrational or emotionally incapacitated, and sometimes moves too precipitously toward exercising the military nuclear option?”

I have been working on precisely such questions for almost a half-century, usually as an independent scholar, but sometimes also as a consultant to certain appropriate agencies of government. In the late 1970s, I was preparing a book on American nuclear strategy and the corollary risks of a worldwide nuclear war, called Apocalypse: Nuclear Catastrophe in World Politics which caused me to become interested in the command-and-control of US strategic nuclear weapons, especially over questions of presidential authority to order the use of such weapons.

Although I quickly learned that there were seemingly reliable safeguards built into all American nuclear command decisions, I also discovered that these redundant safeguards might not apply at the very highest decisional level of all—that is, in the White House.

This disjunction didn’t appear to make any sense, especially in a world where leadership has been known to be irrational—including, in the United States, the case of a prominent nuclear-era Secretary of Defense. (After being institutionalized for assorted and serious psychiatric disorders, US Secretary of Defense James V. Forrestal fell to his death from a naval hospital window on May 22, 1949. Following years of documented clinical depression, it is almost certainly the case that his death was a suicide.)

Concerned about such a readily apparent and potentially fatal lapse in American nuclear planning, I had reached out to retired Gen. Maxwell D. Taylor, a distinguished former Chairman of the Joint Chiefs of Staff. In response to my query, Taylor very quickly sent me a detailed handwritten letter of reply. Dated 14 March 1976, the letter concluded:“As to dangers arising from an irrational American president, the only protection is not to elect one.”

Until now, I had never given any further thought to this informed response, and had simply assumed that “the system,” somehow, would always manage to work. Now, however, with the deeply serious prospect of a President Donald Trump, Gen. Taylor’s 1976 warning takes on a substantially more immediate meaning. It is now at least worth considering that if a President Trump were ever to become unstable or irrational in office, he could then: (1) order the use of American nuclear weapons; and (2) do so with seemingly unassailable decisional impunity.

A precedent? Let us be fair. This is arguably not the first time that the actions of a sitting president or aspiring president should have raised such fearful concerns. (Richard Nixon, in his closing days in office, was hardly an exemplar of emotional stability.) But our national task, going forward, must be to focus purposefully on future threats.

Nixon is long gone; Trump may, however, still be on the verge of arrival.

And there are other considerations.

Even if a President Trump were not unstable or irrational, he might still lack the complex analytical requirements needed to render sober strategic judgments, especially in compressed decision-time periods. For example, during one of the early Republican primary debates, Mr. Trump made plain that he had absolutely no understanding or awareness of this country’s strategic “triad.”

Now, it might be argued that even a presidential nuclear novice would still have ample professional expertise available to him at all times. But as a matter of simple logic, every American citizen should also expect any new president to be familiar with the barest rudiments of America’s strategic nuclear deployments.

I am not speaking here about arcane or otherwise little-known aspects of nuclear strategizing. I am speaking, rather, about what every American high school student is expected to know before he or she receives a diploma. For example, over the many years that I taught international relations at Princeton and Purdue, I correctly assumed that most of my beginning students already understood the most basic elements of the Cold War and US nuclear deterrence policy.

There is more. The United States and Russia are already involved in what amounts to “Cold War 2.0”—a steadily expanding and corrosive development that could complicate any presidential strategic nuclear decisions. Marked by an increasingly bellicose undersea nuclear arms race (submarines), and by accelerating nuclear weapons competition from Moscow, Cold War 2.0 could compel even a well-intentioned and fully-rational President Trump to make ultra high-consequence nuclear decisions in just a few seconds.

In such chronologically compressed circumstances—where seconds matter—what precisely should be done by the National Command Authority, if its members should decide to oppose a “confused” presidential order to use American nuclear weapons? Can the Authority properly respond in a seat-of-the-pants, impromptu, and ad hoc fashion? Or should there already be in place certain measures to suitably vet the sitting president’s reason and judgment—measures of the same sort applied at all lower levels of nuclear command authority?

In principle, at least, any presidential order to use nuclear weapons, whether issued by an apparently irrational president or an otherwise incapacitated one, would have to be followed. After all, for senior figures in the Nuclear Command Authority to do otherwise, and to in any way actually obstruct such an order, would be illegal on its face. In this connection, it should be recalled that candidate Trump has several times advised “killing the families” of alleged ISIS terrorists, and also becoming less constrained by orthodox rules of engagement, i.e., the peremptory rules of Humanitarian International Law. In essence, heeding this advice would be tantamount to reversing authoritative Nuremberg Principles concerning the obligatory disobedience of unlawful orders, and could have uniquely dire consequences wherever such orders would involve nuclear weapons.

The readily imaginable prospect of a Trump presidency is now the most visible manifestation of a deeper structural problem. It follows that any doubts one might currently have about a President Trump armed with the nuclear codes should also be framed as part of a more fully generic discussion of American presidential authority. For example, when faced with a presidential order to use nuclear weapons—and not offered sufficiently appropriate corroborative evidence of an impending existential threat—would the sitting Secretary of Defense or the Chairman of the Joint Chiefs be willing to disobey? And if willing, would one or the other (or some third party) be capable of enforcing any such apparently well-founded expressions of disobedience?

We must finally inquire: Are these important national decision-makers trained or encouraged to exercise private expressions of decisional will? And could they effectively do so without immediately becoming complicit in what would amount to an unwitting American coup d’état?

Significantly, such important questions are merely the tip of the American nuclear command iceberg. Looking ahead, even more specific and detailed questions will need to be asked and answered. If these questions were to be simply avoided, or ignored altogether, we could sometime discover that remediation is already long past due, and that the supposed “only protection” against an irrational American president—“not to elect one,” as Gen. Maxwell Taylor wisely advised—had been too casually disregarded.

Back in the 1960s, a popular movie genre was centered on the assorted risks of nuclear war. Films such as Dr. Strangelove and Fail Safe (both 1964) presented ominous scenarios of an inadvertent nuclear war caused by military command failure, or by mechanical and computer malfunctions. At that time, however, there was never any hint that the critically weak link in the nuclear decision chain could ever be the American president himself.

Now it is conspicuously this highest link that warrants our special concern and appropriately corrective action.

No aspect of the current presidential campaign could possibly be more urgent.