President Donald Trump tweeted last Tuesday that he has a “Nuclear Button” on his desk that is “much bigger & more powerful” than North Korea’s. Except that button doesn’t actually exist.

What Trump does have is something called the “nuclear football,” which is actually a 45-pound aluminum briefcase with instructions for the president on how to launch a nuclear attack. The “football” is always by the president’s side and is carried around by a member of the US military. The president also carries around what’s called the “biscuit,” a card with verification codes that can set the nuclear strike process in motion.

That’s all the president needs to set the process in motion. And he has nearly unchecked power to order a nuclear attack — he doesn’t need to get permission from Congress, the military, or anyone else first. In fact, the military is duty-bound to carry out a lawful order from the president to strike another country with a nuclear weapon.

But if the president were to order a nuclear strike that top military officials considered to be illegal, things could get messy, with people resigning until Trump ultimately found someone to willing to carry out the strike.

So although the actual procedure isn’t quite as simple as Trump just hitting a button, it’s still pretty easy for him to launch a strike if he decides to do so.

How Trump could launch a nuclear strike

Here’s how that system works in the event that Trump decided to attack a country like, say, North Korea with a nuclear bomb:

1) The president decides a nuclear strike is necessary.

It’s unlikely that the United States would turn to nuclear weapons as a first resort in a conflict. There are plenty of nonnuclear options available — such as launching airstrikes to try to take out North Korea’s nuclear arsenal.

But the United States has consistently refused to adopt a “no first-use policy” — a policy not to be the first one in a conflict to use a nuclear weapon, and to only use them if the other side uses them first. Which means Trump could theoretically decide to launch a nuclear strike without the US having been attacked by one first.

Or the US military could detect an incoming nuclear attack from North Korea and the president could decide to respond with a nuclear strike in kind.

Either way, the president is the one who ultimately decides to put the process of launching a nuclear strike in motion if he thinks a strike a necessary. But just because the president thinks a strike is necessary doesn’t mean it automatically happens — he still has a few steps to complete.

2) A US military officer opens the “football.”

Once the president has decided a nuclear strike is warranted, the military officer who is always by the president’s side opens the “football.” The leather-clad case contains an outline of the nuclear options available to the president — including possible targets, like military installations or even cities, that the 900 or so nuclear weapons in the US arsenal can hit — and instructions for contacting US military commanders and giving them orders to launch their missiles.

3) Trump talks with military and civilian advisers.

The president is the sole decision-maker, but he is required to consult with military and civilian advisers before he can officially issue the order to launch a nuclear weapon.

A key person Trump must talk to is the Pentagon’s deputy director of operations in charge of the National Military Command Center, or “war room,” the heart of the Defense Department that directs all US military operations. That’s because the deputy director of operations is the one who actually transmits the president’s orders to the right people in the military.

The president can include whomever else he wants in the conversation. He would almost certainly consult Gen. John Hyten, commander of US Strategic Command, since Hyten is responsible for knowing what the US can hit with its nuclear weapons. But Trump would likely also include Defense Secretary James Mattis, National Security Adviser H.R. McMaster, and Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff in that conversation as well.

The length of the conversation between the president and his advisers is completely up to the president. The chat also doesn’t have to be held in the White House’s Situation Room; it can happen anywhere over a secured phone line.

If any of the advisers felt such an attack would be illegal, they could advise the president against going ahead with the strike. Last November, Hyten publicly said he wouldn’t accept an illegal order from Trump to launch a nuclear attack. "He'll tell me what to do, and if it's illegal, guess what's going to happen?” Hyten told an audience at the Halifax International Security Forum last year. “I'm gonna say, 'Mr. President, that's illegal.’”

He continued by outlining what the military could consider an illegal order: if a nuclear attack isn’t proportional to the actual threat, for instance, or if the attack would cause unnecessary suffering. However, what does and doesn’t constitute a “legal” order is still up for debate and was the focus of a congressional hearing last November.

Either way, if Hyten refused to follow the order, Trump could fire him and replace him with someone who would carry it out.

4) The president gives the official order to strike.

After the conversation, a senior officer in the “war room” has to formally verify that the command is coming from the president. The officers recite a code — “Bravo Charlie,” for example — and the president must then respond with a code printed on the “biscuit,” the card with the codes on it.

Then members of the “war room” communicate with the people who will initiate and launch the attack. Depending on the plan chosen by the president, the command will go to US crews operating the submarines carrying nuclear missiles, warplanes that can drop nuclear bombs, 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,” Joe Cirincione, the president of the Ploughshares Fund, a foundation that works to stop the spread of nuclear weapons, told Vox’s Lindsay Maizland in August 2017.

6) Missiles fly toward the enemy.

It could take as little as five minutes for intercontinental ballistic missiles to launch from the time the president officially orders a strike. Missiles launched from submarines take about 15 minutes.

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