If President Trump wanted to attack North Korea with nuclear weapons, the hard truth is that no one could stop him.

On Thursday, Trump escalated his harsh rhetoric about North Korea by telling reporters that “things will happen to them like they never thought possible” if Pyongyang attacked the US or its Asian allies. The bellicose talk came just days after Trump stunned leaders around the world by promising to hit North Korea with “fire and fury” if it continued to threaten the US.

For its part, North Korea announced plans to fire four ballistic missiles toward the US territory of Guam, which houses 6,000 American troops. Pyongyang often issues such threats without following through, but North Korean missiles are notoriously imprecise, which means there’d be a real chance of one of those missiles landing on Guam. If it did, some form of military confrontation between the US and North Korea might be inevitable.

It’s important to take a deep breath. The North Korean regime isn’t irrational, and the ruling Kim family has spent decades working to guarantee it maintains power. It’s difficult to imagine Kim Jong Un, the current ruler, launching a nuclear missile at the US when he knows the US response would erase his country from the map.

It’s also hard to imagine Trump, for all of his tough talk, actually giving an order that would lead to millions of deaths.

But if he did give the order, the military would be duty-bound to carry it out. Trump, and Trump alone, is vested with the power to order the use of the most destructive weapons the US possesses.

“We have a nuclear monarchy,” said Joe Cirincione, the president of the Ploughshares Fund, a security foundation that tries to stop the spread of nuclear weapons, and the author of Nuclear Nightmares: Securing the World Before It Is Too Late. “Once he gives the command, he cannot be overruled.”

If Trump did give the command — a very big if — here’s what would happen between the time the president made the decision and the time the missiles started to fly:

1) The president receives word of an incoming attack.

Experts I talked with said it’s unlikely that either North Korea or the United States would turn to nuclear weapons, especially as a first resort. There are other, nonnuclear options that both countries would consider first, such as sending artillery across the Korean border or launching airstrikes.

“Nuclear war is not about to break out, but [Trump’s] language increases the risk of miscalculation,” said Daryl Kimball, the executive director of the Arms Control Association, a nonpartisan organization dedicated to public education on arms control. “And when it comes to the Korean Peninsula, when there’s a miscalculation in war, then there’s a possibility of nuclear war.”

That war would begin with Trump receiving word of an imminent North Korean attack.

2) The nuclear briefcase, or “football,” is opened.

After the president learns of the attack, the military officer who is always by the president’s side opens a briefcase known as the “football.” The black case contains an outline of the nuclear options available to the president and instructions for contacting US military commanders around the world and giving them orders to launch their missiles.

3) There’s a conversation with two top military officers.

As mentioned earlier, the president is the sole decision-maker. But he must consult two people to make that decision.

He must talk with the Pentagon’s deputy director of operations in charge of the National Military Command Center, or “war room,” the heart of the Pentagon from where all US military operations are directed. The current deputy director is Lt. Gen. John Dolan. The president must also speak with the commander of US Strategic Command, currently Gen. John Hyten.

The length of the conversation depends on the president. It also doesn’t have to be held in the White House’s Situation Room; it can happen anywhere over a secured phone line.

The president can include whomever else he wants in the conference, meaning son-in-law Jared Kushner could in theory help Trump decide whether to use nuclear weapons. He would almost certainly include Defense Secretary James Mattis and the Joint Chiefs of Staff.

But the only people who really matter are the deputy director of the National Military Command Center and the Strategic Command commander. The officers could try to convince the president not to launch an attack. They could resign on the spot in protest. Even if they did, though, they would most likely be replaced by officers willing to issue the command to strike.

4) The president makes the decision, and the order is given.

To verify that the command is coming from the president, the officers recite a code (“Bravo Charlie,” for example). The president must then respond with a code printed on a card that he carries with him at all times, known as the “biscuit.”

Then the two officers communicate with the people who will initiate and launch the attack. Depending on the plan chosen by the president, the command will go to navy crews operating the submarines carrying nuclear missiles or troops overseeing intercontinental ballistic missiles on land.

5) Launch crews prepare to attack.

The launch crews receive the plan and prepare for attack. This involves unlocking various safes, entering a series of codes, and turning keys to launch the missiles. The crews are trained to “execute the order, not question it,” said Cirincione.

6) Missiles are launched.

It could take about five minutes for intercontinental ballistic missiles to launch from the time the president announced his order. Missiles launched from submarines take about 15 minutes.

The whole process is designed to be fast because if missiles are heading toward the United States, they could land within 30 minutes. If the president chose to, he could launch US missiles before the enemy ones hit.

“The president can order a nuclear strike in about the time it takes to write a tweet,” said Cirincione.