President Trump at the White House with Defense Secretary James Mattis in October. (Leah Millis/AP)

The abrupt and pointed resignation of Secretary of Defense Jim Mattis on Thursday alarmed official Washington. Sen. Mark R. Warner (D-Va.) called him an “an island of stability amid the chaos of the Trump administration.” Retiring Sen. Jeff Flake (R-Ariz.) told The Washington Post that “having Mattis there gave all of us a great deal more comfort than we have now.”

Mattis’s departure seems to be provoking unease, especially considering how dangerous our nuclear-command arrangements are. The notion that Mattis, a former four-star Marine Corps general, could have blocked or defied a move by Trump to impulsively launch nuclear weapons may have seemed comforting, but it shouldn’t have been. The secretary of defense has no legal position in the nuclear chain of command, and any attempts by a secretary of defense to prevent the president from exercising the authority to use nuclear weapons would be undemocratic and illegal. With or without Mattis, the president has unchecked and complete authority to launch nuclear weapons based on his sole discretion.

The reaction to Mattis’s resignation, however, could open the door for the new Congress to create long-overdue legal barriers preventing the president from initiating a nuclear strike. Such a step could be implemented without any negative impact on U.S. security or that of our allies.

Every day, the U.S. nuclear early warning system is triggered by some event or another, mostly civilian and military rocket launches by one or more of a dozen countries with ballistic missiles. When such launches appear to threaten North America, the head of U.S. Strategic Command is alerted, and sometimes these alerts warrant the urgent notification of the president. That alert comes by way of a direct call from the Strategic Command or via the White House Situation Room, the emergency-operations bunker beneath the East Wing, or the national security adviser. Partly a remnant of the Cold War, this system remains in place today to ensure the president can be notified quickly of any direct threat to the United States’ nuclear arsenal and the facilities that control it. That way, he can launch nuclear missiles before they are destroyed or the U.S. government is incapacitated by incoming weapons.

In normal times, this system is precarious, and it can pressure even experienced leaders to consider nuclear weapons in a crisis sooner than warranted. Alerts stemming from ambiguous ballistic nuclear missile threats occurred multiple times during the administrations of George W. Bush and Barack Obama, and some alerts went directly to those presidents.

Yet, this system seems especially ill-suited to a president who has demonstrated time and again that he can be provoked into taking rash action, and who, as a candidate, openly questioned why the United States could not use the nuclear weapons it possesses. This is a dangerous set of instincts for a commander in chief with sole and unchecked authority over almost 4,000 nuclear weapons, nearly 1,000 of which could be fired within a few minutes.

For over a year, Mattis has been trying to reassure congressional leaders that he could help check some of Trump’s impulses, in part by intervening in the nuclear chain of command. In a break with normal procedures, Mattis reportedly told the commander of the Strategic Command to keep him directly informed of any event that might lead to a nuclear alert being sent to the president. He even told the Strategic Command “not to put on a pot of coffee without letting him know.”

Congressional leaders interpreted this to mean that Mattis would either deal with a possible threat before it reached Trump or ensure he was present to advise Trump when such an alert arrived.

This assurance may have helped ease concerns about our nuclear weapons for some members of Congress, but only if they were unfamiliar with how the command and control structure truly works. Personal relationships and back channels are no way to manage a nuclear arsenal.

Even informed observers are surprised to learn the president can order the use of nuclear weapons without the input — or consent — of the secretaries of Defense or State, the chairman of the Joint Chiefs of Staff or the vice president. They only have a role in the presidential launch protocol if the president has given prior approval for them to be notified and solicits their advice. Otherwise, none of these people would need to be involved or informed that the president has decided to use a nuclear weapon.

Under standard procedure, an attempt would be made to contact key national security officials, but in some real-world and exercise scenarios, it has proven impossible to tie them into a quickly convened emergency teleconference. Should he wish, the president could exclude all of them, and even bypass the primary designated adviser — the four-star general in charge of U.S. strategic forces — by ordering a low-ranking on-duty emergency operations officer at the Pentagon or elsewhere to transmit a launch order directly to the executing commanders of strategic U.S. submarines, silo-based missiles and bombers.

Trump could have learned all this in a briefing about nuclear weapons shortly after he took office, and his military aide, ever at his side, could explain and assist in issuing a direct order to a lower-level officer at any time.

Even if Mattis had been with Trump at a time of nuclear crisis, his resignation letter drives home the fact that Trump might very well have simply ignored his counsel. Trump, as he is proving in stark terms, listens only to himself. And any attempt by another person to physically block the president from issuing a launch order would probably result in his or her removal by the Secret Service. It is delusional and fundamentally undemocratic to think that our strongest check on a president bent on initiating nuclear war without justifiable cause might be a defense secretary trying to keep the president from communicating his launch authority using the so-called Gold Codes.

When the United States faced the prospect of sudden nuclear attack from the Soviet Union, this system helped reinforce deterrence based on a balance of nuclear terror. But since the demise of the U.S.S.R., and even with a more aggressive Russia, the whole arrangement raises questions about its necessity, risks and consistency with democratic values. It is well past time for the system to be reformed to ensure that it hews to our Constitution and mitigates as much as possible the very real risks associated with a renewed arms competition with Russia.

One key issue is whether Trump — or any president — should have the legal ability to independently initiate the use of nuclear weapons. It seems reasonable that the president needs to be able to quickly order a nuclear response if an adversary employs nuclear weapons first against us, and that he would not have time to consult with Congress or the Cabinet if nuclear missiles were headed here. (The flight time of ballistic missiles over intercontinental distances is 30 minutes or less, and the president would have only about five to seven minutes to decide whether and how to respond.)

However, our chain of command is not just a presidential preference — it can be determined by legislative action. Congress can and should prohibit any president from using nuclear weapons first. The incoming chairman of the House Armed Services Committee, Rep. Adam Smith (D-Wash.), proposed such legislation last year. It states that it is the policy of the United States not to be the first to use nuclear weapons. Congress could make any first-use illegal, constraining the president from issuing such an order and obligating any member of the military to disobey a command to do so. A no-first-use policy would also ratchet down tensions with Russia and facilitate reductions in the number and types of nuclear weapons in both U.S. and Russian arsenals. The logic and political salience of this position is growing, with some 20 members of the incoming Congress — including House Speaker-to-be Nancy Pelosi (D-Calif.) — now on record supporting no first use.

Legislation to bar first use probably wouldn’t get through the Republican Senate or be signed into law by Trump. But recognition that the system puts too much power in the hands of one person increases the likelihood that the next president will either adopt such a posture or accept legislative controls. Maintaining an outdated and unstable system is clearly too dangerous.

Bending norms and the military chain of command to prevent a disastrous presidential decision is not a reliable safeguard, and extralegal measures should not be how the United States prevents a nuclear war. Neither Mattis nor anyone else can reassure the American people that a president will not, on a whim, use the most fearsome weapons humans have ever invented. Only laws can constrain such a dangerous prospect. It is well past time for our country to take control of the nuclear chain of command.

Read more:

The president would probably never order the use of nuclear weapons

This is how nuclear war with North Korea could unfold

A new Trump administration policy makes nuclear war more probable