Article by Tim Marshall, bestselling author of Prisoners of Geography and former foreign and diplomatic editor of Sky News
It's very tempting to think that North Korea's leader Kim Jong-un has a big red button on his desk akin to that of Dr Evil in the Austin Powers films. This image has been conjured up recently by the threat and counter-threats of Kim and President Trump and the size of their respective buttons.
In his evil lair, Dr Evil hovers a finger over the button and on a whim, presses it, unleashing death and mayhem of one sort or another. However, it's unlikely that in Kim's nuclear weapons system that this is the case, although, as we know so little about the Hermit Kingdom, it can't be ruled out.
A more likely scenario is a strict chain of command series of orders and actions akin to that in the American system which culminates not in a button being pushed, but keys turned. It's worth looking at the American protocols before comparing them to the UK to see what buttons Prime Minister May would push. Spoiler alert – none.
In the United States, the President has the sole authority to order a nuclear strike.
Here's the likely scenario: The President meets senior military advisors in the Situation Room in the White House. If he's travelling a call is made on a secure line. If he still wants to go ahead a 'challenge code' is read to him. It's usually two phonetic letters, eg – 'Delta Tango'.
The President receives the 'biscuit', a laminated card from which he reads aloud the matching response to the code. An aide, always close to the president, will have the 'football', a briefcase containing the 'Sealed Authentication System' (SAS) codes which are required to launch missiles.
The Pentagon transmits these to the senior officers at the bases or submarines from where the missiles are to be fired. They unlock safes containing physical keys and compare their SAS codes with those received.
If fired from land, five launch crews in different locations simultaneously turn a key. To protect against mutiny, only two of the five are required to be turned for the missiles to launch. From land, the whole process could take as little as five minutes. For a sub launch, it could be within 15 to 20 minutes.
So, that's Donald's 'button'. How big is it? It's huge. The USA has about 6,800 warheads and as far as is known, as he says, they work. The North Koreans have an unknown number. Estimates range from 10 to 20 and it's still not known if they have a capable delivery system.
In the event that Prime Minister May decided to launch one of the UK's estimated 215 warheads, she too would initiate the process which would then be passed down the chain of command.
However, as far as is known, unlike in the American system, there is a scenario in which UK missiles could be fired without the go-ahead from a politician.
The UK has four nuclear submarines, each carrying 16 Trident missiles, and at least one sub is on patrol at all times. The UK does not have an airborne nuclear deterrent.
The UK decision to fire would probably be made from the air-locked Defence Crisis Management Centre, a bunker under the Ministry of Defence which this writer has visited. It is a spartan complex of tunnels and rooms several storeys down. It was built in the 1990s and there's a late 20th century feel to most of it, with basic desks and wooden bunk beds. But some of the technology down there is state-of-the-art.
Some officials call the complex 'Pindar', after the Ancient Greek poet whose house was the only one spared when the Macedonians destroyed the city of Thebes. Given that the UK deterrent is only envisaged being used in response to a nuclear attack, the scenario here is that after a first strike the PM, cabinet ministers and military personnel would be gathered in Pindar.
If the Prime Minister decides to retaliate orders are relayed to Northwood Headquarters under the Chiltern Hills, and from there to the UK's submarines. At each stage, the message must be confirmed by two people.
The submarine commander, or commanders if more than one is at sea, then initiate the launch sequence, which culminates in a red device with a trigger being picked up (see top). It is modelled on the handle of a Colt.45 peacemaker handgun. When the trigger is pulled, the missiles are fired.
The only scenario whereby the trigger is pulled without orders from a politician is if the first strike against the UK has left no-one with the authority to give the orders to fire back. After a series of checks over several days, the commander of the submarine at sea will then open a safe containing a 'Letter of Last Resort'.
Every Prime Minister, at the beginning of their term in office, writes such a letter containing their final instructions. The options are to retaliate, to not retaliate, to place the vessel under the control of the US or Australian Navy, or, for the decision on what to do to be handed to the commanders. These letters are destroyed at the end of each Prime Minister's time in office.
There are reports that it matters not what size, colour, or design, the UK Prime Minister's button is because the Americans control the UK deterrent. This is not entirely true.
Technically the UK Prime Minister can give the order to fire and the missiles can be launched. They do not rely on any American politician, military officer, or technology to be fired and to reach their target (the US GPS system can be used for targeting but is not essential).
This is why the then-Europe Minister David Lidington was able to tell Parliament in 2014 that "Only the Prime Minister can authorise the employment of the United Kingdom's nuclear deterrent and there are no technical means by which the United States could negate or override a prime ministerial instruction."
However, the political reality is that it is very unlikely that any British PM would launch a nuclear missile without consulting with the Americans first.
In the event of the Americans saying no, and the British going ahead, or, in the event of a massive rift in the 'Special Relationship' the USA can simply turn off the technological taps required for the British deterrent to function.
The UK's Trident missiles are leased from the USA. At the most basic level, the USA could refuse to maintain the missiles when they are returned to Kings Bay, Georgia for servicing, and refuse to lease any new ones.
It is the case that in the heat of the moment the UK can launch a nuclear missile, but it is also the case that in almost all realistic scenarios, the USA has the political and technological clout to prevent it. You could lease a car from Hertz, then deliberately drive it over a cliff. Even if you survive, it's unlikely Hertz will rent you another one.
There are no buttons, but there are keys and triggers, both physical and metaphorical.
Cover image courtesy of Danny Lawson/PA Archive/PA Images.