If President Donald Trump decided to authorize the use of nuclear weapons in a military conflict with North Korea, a possibility he has left open amid escalating tensions, it appears there’s little that could stop him.

The prospect of a nuclear flashpoint comes after a war of words between Trump and the North Korean government, with the president warning this week of “fire and fury” against Pyongyang if it continued making threats, and North Korea subsequently threatening an attack on the U.S. territory of Guam. Trump on Thursday suggested that his initial statement may not have been tough enough, and tweeted Friday that the U.S. military was “locked and loaded” should North Korean “act unwisely.”

Despite Trump’s bellicose rhetoric, diplomatic options are still available. And it’s far from certain that he would ever actually give an order for a nuclear strike.

But if he did, there’s little legally or procedurally to block him. While details of the process for launching a nuclear attack are largely kept secret, the delegation of authority is clear.

“There’s no veto once the president has ordered a strike,” Franklin Miller, who worked in the White House and Pentagon on nuclear issues, told the New York Times last year. “The president and only the president has the authority to order the use of nuclear weapons.”

The military employees charged with executing the president’s order could, theoretically, refuse to carry out the president’s order, but would risk a court martial, according to Michael O’Hanlon, a senior foreign policy fellow at the Brookings Institution.

“A president could push the button all by himself or herself, legally-and constitutionally-speaking. Physically, military personnel would need to carry out the strike of course. They could choose not to, perhaps at the instruction of the secretary of defense or the four-star officer leading Strategic Command — who together constitute the chain of command between the president and the trigger-pullers. But any military officer ignoring a presidential order would be in open insubordination, subject to dismissal and court martial,” O’Hanlon wrote in a blog post last year.

Alex Wellerstein, a historian of nuclear weapons at the Stevens Institute of Technology, said in a Washington Post op-ed last year that it’s unclear if the secretary of defense could block a nuclear attack.

“The chain of command as widely understood requires that the president order the secretary of defense to carry out a launch; the secretary serves as the conduit for implementation by the military,” he wrote. “Could the secretary of defense refuse to carry out a presidential order for a nuclear attack? The legal and constitutional aspects are not clear. The official doctrine that has been released says nothing about this question, and the cryptic public responses to official inquiries, even from Congress, indicate that it is not something that can be openly talked about.”

“Could the president simply fire the defense secretary and move on to the deputy secretary, the secretary of the Army and so on through the chain of command? Maybe. Such an action would at least slow things down, even if the refusal to carry out the order was illegal,” Wellerstein continued. “But the secretary may not even be formally required to participate — U.S. Air Force doctrine does not indicate he is a necessary part of the chain of command, and holds that the president can communicate directly with the military, in the form of the Joint Chiefs of Staff, to order a nuclear strike.”

According to Vox, Trump would also have to speak with the Pentagon’s deputy director of operations and the commander of U.S. strategic command before an attack was launched. Those officials could try and convince Trump not to launch an attack, or resign. If they resigned, they’d likely be quickly replaced with officers who would carry out the president’s orders.

In the past, there have been instances where military officials have questioned a U.S. president’s ability to launch an attack or quietly tried to block it.

In 1969, President Richard Nixon wanted to put U.S. nuclear forces on high alert in order to attempt to scare the Soviet Union into thinking the U.S. was prepared to use nuclear weapons in the Vietnam War. Nixon’s Secretary of Defense, Melvin Laird, tried to delay the plan from going into action, citing ongoing readiness exercises, but was ultimately unsuccessful, according to the Times.

Five years later, in the days leading up to his resignation, Nixon’s secretary of defense, James Schlesinger, ordered that any nuclear order be cleared by him or Henry Kissinger, the secretary of state, the Times reported. The order may have been illegal, but aides at the time were concerned with Nixon’s stability and drinking and it went unquestioned.

Even questioning the president’s authority to launch a nuclear attack can lead to trouble. In 1973, Harold Hering was training to be a missile launch officer when he asked his teacher how he could be sure an order to launch a nuclear attack was lawful.

Hering told Radiolab in April he felt he was being asked to assign “blind faith values” to the president, and recalled asking what a person should do if a president giving the order wasn’t sane. Hering was dismissed from the military for even asking the question because supervisors didn’t think he could be trusted to follow the chain of command.

Congress intended for the president to have the authority to act alone when it came to nuclear weapons, according to Wellerstein. The president’s nuclear power was codified in the Atomic Energy Act, which President Harry Truman signed in 1946.

“The members of Congress who wrote the law, largely with the backing of the scientists who worked on the Manhattan Project, framed it explicitly as a question of who controls the power to use nuclear weapons: Is dropping an atomic bomb a military act or a political one?,” Wellerstein wrote in The Washington Post in December. “If it is inherently political, above and beyond a regular military tactic, then that power could not be entrusted to the military. Ultimately, the president was supposed to be the check against the Pentagon pushing to use nukes more often.”