If President Trump wanted to fire nuclear weapons, he could first discuss a range of options with his advisers.
He'd then issue an order to top military officials at the Pentagon.
They would confirm the president's identity by exchanging codes printed on a card called the "biscuit", which the president carries wherever he goes.
The order would pass to US Strategic Command, which would then send further instructions and access codes to crews on the ground (or, perhaps, underwater in a submarine.)
Those crews would then launch the missiles.
The BBC's Inquiry podcast covered the practicalities of launching a nuclear strike in much more detail.
Is there anyone, at any of those stages, who'd be allowed to say "no" to the president?
Chain of command
A briefing from the Congressional Research Service makes the legal situation plain. "The US president has sole authority to authorise the use of US nuclear weapons."
Ordinarily, nobody is allowed to over-rule the president's decision - it's part of his role as Commander-in-Chief.
In theory, the vice-president could oust the president if a majority of the cabinet agreed that the president was unfit to serve.
In practice, that would be difficult to organise in time to stop the president launching nuclear weapons.
But Peter Feaver, professor of political science at Duke University in North Carolina, says it's not true that President Trump could launch a nuclear strike as easily as he could fire off a tweet.
"The president is giving an order which is transmitted down a chain of command. Someone further down that chain of command turns the key or presses the button."