This piece is adapted from Fred Kaplan’s new book, The Bomb: Presidents, Generals, and the Secret History of Nuclear War.

As senators try President Donald Trump for impeachment and some of them call for placing limits on his ability to wage war against Iran, it is worth recalling that, early on in his term, lawmakers of both parties raised fearful concerns about Trump’s war powers more broadly—specifically whether he should have the power to start a nuclear war all on his own.

On Oct. 30, 2017, the Senate Foreign Relations Committee held a hearing on whether the president needed new congressional authorization to use military force against terrorists around the world. When his turn came to ask questions, Democratic Sen. Edward Markey asked the witnesses whether Trump could launch a nuclear first strike without consulting anyone from Congress.

At first, the witnesses, Secretary of State Rex Tillerson and Secretary of Defense Jim Mattis, hesitated, calling the question “hypothetical,” but Markey wouldn’t relent, and finally, Mattis allowed that the president could order a first strike if an adversary was seen “preparing” to launch an attack.

Markey, a longtime advocate of nuclear arms treaties, knew the answer before asking the question, but some of the senators were surprised. Among them was the Republican chairman, Bob Corker. A businessman from Tennessee, Corker was deeply conservative, but he was also agitated by stories he’d been hearing about Trump’s mental state. Recently Corker had made a stir by likening the White House to an “adult day center” and warning that Trump’s reckless threats toward other countries could pave a “path to World War III.”

After the hearing, Corker told his staff that he was “riled up” by Markey’s exchange with the two secretaries and that he wanted to hold a separate hearing on the subject as soon as possible—“something real sober,” as he put it, “pointing out that the president has the power to basically destroy the world.”

The hearing was held just two weeks later, on Nov. 14, the first such hearing on the subject in 41 years. The staff assembled three witnesses—one Democrat, one Republican and one retired four-star general. The general was C. Robert Kehler, former head of U.S. Strategic Command, which is in charge of plans and policies on nuclear weapons.

Kehler testified that the president did not have a completely free hand. “The United States military does not blindly follow orders,” he said. “A presidential order to employ U.S. nuclear weapons must be legal. The basic legal principles of military necessity, distinction, and proportionality apply to nuclear weapons, just as they do to every other nuclear weapon.”

Sen. Benjamin Cardin, the ranking Democrat on the panel, asked who would decide whether the order was illegal.

“Well,” Kehler replied, “that is one of the things that would be on the plate of the commander of Strategic Command.” If he thought the order was illegal, he would be “obligated to refuse to follow it.”

But on further questioning, Kehler’s bold assurance turned wobbly. Republican Sen. Ron Johnson asked him how, as Stratcom commander, he would have gone about refusing to follow an illegal order.

Kehler replied, “I would have said, ‘I have a question about this,’ and I would have said, ‘I’m not ready to proceed.’ ”

“And then what happens?” Johnson asked.

“Well,” Kehler said. He paused, and nervous laughter flitted through the hearing room. “As I say,” he went on, with a slight grin, “I don’t know exactly. Fortunately, we’ve never—these are all hypothetical scenarios. I mean, they’re real, in terms of—”

Johnson interrupted: “We are holding a hearing on this, so—”

“Exactly,” Kehler replied. “This is the human factor in our system. The human factor kicks in.”

And that opened the door to the question of what happens to the legal principles of war when the human is Donald Trump.

Democratic Sen. Chris Murphy made the point in the starkest terms. “Let me just pull back the cover for a minute from this hearing,” he began. “We are concerned that the president of the United States is so unstable, is so volatile, has a decision-making process that is so quixotic, that he might order a nuclear-weapons strike that is wildly out of step with U.S. national security interests. Let’s just recognize the exceptional nature of this moment in the discussion that we are having today.”

Remarkably, no one—not even any of the committee’s Republican senators—challenged this charge.

Murphy then asked the Republican witness, Peter Feaver, who had been a special adviser to President George W. Bush, whether an adversary’s mere possession of a nuclear weapon, with the range to reach the United States, would constitute an “imminent attack” and thus put the president on solid legal grounds in ordering a first strike.

Feaver, while noting that he wasn’t a lawyer, replied that it probably did pass the legal test, saying, “I think it would, in most people’s minds, constitute a grave threat to U.S. national security, particularly if it was a North Korean nuclear warhead atop a North Korean missile that was capable of reaching the United States.”

Brian McKeon, the Democratic witness, who had worked in the National Security Council and the Pentagon under President Barack Obama, disagreed. “Senator,” he said, “the mere possession of a nuclear weapon, I do not think would meet that test.” After all, the North Koreans “have a nuclear weapon today, we know that much.” If the president considered that fact sufficiently threatening to warrant a U.S. first strike, there would be plenty of time to seek congressional authorization.

As the hearing went on, it became clear that the debate was ultimately beside the point. Kehler noted that, in deciding whether to launch nuclear weapons, the president would consult with many officials—the secretary of defense, head of Strategic Command, and others, any one of whom could say, “Wait, stop, we need to resolve these issues.” But, he finally acknowledged, the “decision authority”—the legal power to launch nuclear weapons—“resides with the president.”

Markey, who had first raised the issue, made what he considered the central point:

Absent a nuclear attack upon the United States or our allies, no one human being should have the power to unilaterally unleash the most destructive forces ever devised by humankind. Yet under existing law, the President of the United States can start a nuclear war without provocation, without consultation, and without warning. It boggles the rational mind. I fear that, in the age of Trump, the cooler heads and strategic doctrine that we once relied upon as our last best hope against the unthinkable seem less reassuring than ever.

The hearing came to a close after a few hours—by contrast, 1976 House hearings on the same subject lasted four days—with no conclusions or consequences. A bill introduced by Markey, to give Congress a say on approving an unprovoked first strike, had no more chance of passing than similar bills put forth in the past. (In 1972, Sen. William Proxmire proposed an amendment to the War Powers Act, barring the president from launching a nuclear first strike without congressional approval. It was voted down, 10–68.) Few in Congress have ever wanted the responsibility of making weighty decisions on war and peace. Few Republicans at the 2017 hearing even took much interest in exploring the dilemmas. Most of them used their time not to ask probing questions but to warn against the dangers of letting—as Sen. Marco Rubio put it—“a bunch of bunker lawyers decide that they are going to disobey any order that they disagree with.”

Kehler was flustered by the whole proceeding. To his mind, it was up to Congress to set the rules on the procedures and safeguards for the use of nuclear weapons. If the members of Congress thought the president was unfit to command, then they needed to take responsibility and change the procedures. They couldn’t just toss it in the laps of the military. Worse still would be if they raised provocative questions about the president’s fitness then did nothing, leaving the generals with a chain of command that no one trusted. But this, he thought to himself, was exactly what this committee was doing.

There was a history of senior officials and underlings maneuvering around an untrustworthy chain of command. Back in the summer of 1974, amid reports of President Richard Nixon’s frequent drunkenness under the pressures of Watergate and his imminent impeachment, Secretary of Defense James Schlesinger quietly asked the Joint Chiefs of Staff to call him if they received any “unusual orders” from the president. (Neither Schlesinger nor the chiefs, then or now, were in the chain of command for nuclear orders, so this would have technically been an act of insubordination.)

In late 1973, Maj. Harold Hering, a Minuteman missile launch officer in training, asked his instructors, “How can I know that an order I receive to launch my missiles came from a sane president?” And, more broadly, “What checks and balances exist to verify that an unlawful order does not get in to the missile men?” Hering, a proud Air Force officer, had served multiple tours as a helicopter pilot in Vietnam. He simply wanted assurances that, if he ever got the signal to launch nuclear missiles against a foreign country, he would be following legal orders as military law required. For his devotion to the law, he was instantly yanked out of missile crewman class, given a desk job, and, after a review board meeting, drummed out of the military.

Nobody wanted to answer Hering’s questions, in part because they couldn’t be answered without raising doubts about the whole system of command and control over nuclear weapons. They aroused suspicions that the elaborate process of consultation over the decision to launch nuclear weapons might be fragile. “The human factor kicks in,” as Kehler testified. There was no safety switch in place, no circuit breaker that someone could throw, if the human in charge turned out to be crazy.