President Trump speaks to U.S. military personnel at Yokota Air Base in Tokyo. (Jim Watson/AFP/Getty Images)

IMPULSIVE, BOMBASTIC and prone to grudges, President Trump has stirred serious questions in the minds of many Americans about the command and control of nuclear weapons. Mr. Trump has already threatened North Korea with “fire and fury like the world has never seen.” Could he really do it? Are there restraints? The Senate Foreign Relations Committee under Chairman Bob Corker (R-Tenn.) deserves credit for airing this timely concern in a hearing this month, the first of its kind since 1976.

Naturally, the questions are being driven by Mr. Trump’s personality and might not have been raised had someone else been elected president. But the fact is that Mr. Trump sits in the Oval Office and holds the power to launch nuclear weapons, and an understanding of how that works is important.

Start with a key distinction between today and the Cold War. During the long confrontation with the Soviet Union, both sides threatened each other with nuclear-armed missiles that could hit the other in 30 minutes. Both nations built elaborate command-and-control systems to enable a rapid response to attack. The threat of certain and rapid retaliation — the cocked-pistols standoff — was intended to deter war. To effectively deter, it had to be credible. In the United States, the president was, and is, the sole decider. He carries a card to authenticate his authority, and in the event of an emergency attack on the United States, with the help of military aides, he would have to make excruciating decisions in just minutes: whether the nation is under attack, how to retaliate, what targets to aim at, what weapons to launch. Ground-based missiles can be launched within about four minutes of the president’s order, sea-based within 12 minutes or so. Obviously, in such an emergency, there is no time for a Cabinet meeting or consultation with Congress. This system still exists.

But the anxiety felt by most Americans is largely not about the Cold War scenario. Rather, it is about a nuclear conflict in today’s world of second-tier nuclear powers, particularly North Korea, run by another bombastic leader, which has conducted six nuclear tests over 11 years and is working on long-range missiles to deliver nuclear warheads. The worry is that any kind of conflict — perhaps a war started with nonnuclear forces, or a sudden missile launch, or a misunderstanding — might prompt the impulsive Mr. Trump to demand a nuclear strike. In this scenario, as with the Cold War attack, Mr. Trump’s authority at the apex of the command-and-control pyramid is not in doubt. But there are restraints.

Most likely, there would be somewhat more time for deliberation, a window for others to scrutinize and possibly dispute or delay a nuclear launch order. Hans Kristensen of the Federation of American Scientists obtained 2012 U.S. strategic war plans under the Freedom of Information Act that clearly state that strike options and execution must pass muster under domestic and international law and that any weapon use “must comply” with requirements of the Law of Armed Conflict: “military necessity, avoidance of unnecessary suffering, proportionality and discrimination or distinction.” The former head of U.S. Strategic Command, retired Air Force Gen. C. Robert Kehler, testified that the U.S. military does not blindly follow orders and would question — and check — whether an order to launch nuclear weapons was legal. The Senate panel was also reminded that the system is made of people, processes, checks and controls and is not automatic. Furthermore, when it comes to fighting wars in general, Congress shares authority with the chief executive, and that authority might be exercised more clearly in a nuclear use decision not taken under emergency conditions.

Still, there are disturbing gray zones. What if conventional war breaks out, say, on the Korean Peninsula, Congress gives approval to defend U.S. allies South Korea and Japan, casualties soar and Mr. Trump wants to use a nuclear weapon in the hope that it would quickly end the conflict? He might argue that a low-yield nuclear weapon aimed at a remote North Korean weapons target is proportionate, militarily necessary and able to halt further suffering. What is to stop a president then? The boundary between conventional and nuclear war could be very blurry, as it was in Europe during the Cold War. These questions bear further scrutiny.

Mr. Trump’s personality does not seem likely to change. But there is one area where he could make the world safer: the Cold War procedure, in effect in the United States and Russia, of keeping land- and sea-based nuclear missiles on launch-ready alert. This is a holdover from the era of mutual assured destruction that could be modified in tandem by the United States and Russia, a smart, pragmatic move to ease off the hair-trigger alerts, which pose a threat of miscalculation and catastrophe. The nuclear weapons would still retain their awesome destructive power; they would remain a potent deterrent. But giving a leader more than mere minutes to decide whether to launch them in a crisis seems to be a wise step that Mr. Trump, who carries that nuclear weapons authentication card around with him, can surely appreciate.