DONALD Trump, as President of the United States and Commander-in-Chief of its military, has the most devastating nuclear arsenal in history at his disposal.
Trump has made frequent references to America's nuclear capability and has famously said the power he wields is "very, very scary". But is he the only man authorised to press the button?
Being given access to the launch codes is one of the first tasks of any new Commander-in-Chief.
Reports say he will always carry the so-called Gold Codes on his person, printed on a credit-card sized piece of plastic nicknamed "the biscuit".
If he is away from the White House, he will be accompanied by a military aide who carries the "nuclear football" – a briefcase containing a mobile hub linked to the strategic defence system.
Once a launch has been ordered, a short code is sent to sites where crews will authenticate them before launching.
Trump has denied he would recklessly launch nuclear action, but on his election many raised the fact he would have access to powerful weapons as a big worry.
If President Trump were to give the order to attack, no one can stop it – the military will simply confirm the codes and fire.
Officers may resign, but the attack would still go ahead.
Bruce G. Blair, a former Minuteman missile-launch officer and research scholar at Princeton told Bloomberg: "The Commander-in-Chief’s power is clear: He or she has sole authority to use nuclear weapons.
"Before initiating military action, the president convenes a conference with military and civilian advisers in Washington. If travelling, the President is patched in on a secure line.
"The consultation lasts as long as the president wishes, but if enemy missiles are heading toward the US and the president must order a counter strike, the consultation may last just 30 seconds."
Once he has ordered a launch, a short code is transmitted to launch sites – where crews will then authenticate them against sealed codes provided by the National Security Agency, before launching.
Donald Trump could have his unilateral power to fire a nuclear missile revoked.
The US Senate was due to discuss on 15 November 2017 whether the President could be trusted with such a daunting ability following his brash "fire and fury” threats against North Korea.
“This discussion is long overdue,” Senator Bob Corker, a staunch Trump critic, said in a statement announcing the hearing, which was to be held by the Senate Foreign Relations Committee.
The scheduled hearing would be the first time since 1976 that Congress “have looked specifically at the authority and process for using US nuclear weapons,” Corker said.
Defence Secretary Jim Mattis played down Mr Trump's role in nuclear launch decision-making, telling reporters: “I’m the President’s principal adviser on the use of force.”
Asked whether he was happy with the current system as it was, Mattis simply said: “I am.”
America currently has about 900 active nuclear warheads.
The strategic nuclear warheads are deployed on intercontinental ballistic missiles (ICBMs), submarine-launched ballistic missiles (SLBMs), and strategic bomber aircraft.
The State Department announced in April 2015 that approximately 2,500 warheads are retired and await dismantlement.
The United States and Russia account for 93 per cent of nukes. Since their peak in the mid-1980s, global arsenals have shrunk by more than two-thirds.
More countries have given up weapons and programs in the past 30 years than have tried to acquire them.
In his first TV interview as President, Trump said he would have no qualms about launching a nuclear strike.
He told ABC’s David Muir: “When they explain what it represents and the kind of destruction that you’re talking about, it is a very sobering moment.
"It's very, very, very scary in a sense."
He added: "I have confidence I'll do the right thing, the right job, but it's a very scary thing."
Trump has gone back and forth on the nuke issue, but had suggested while campaigning that South Korea and Japan should be allowed to acquire nuclear arms.
He said: "It's not like, gee whizz, nobody has them. So, North Korea has nukes. Japan has a problem with that.
"I mean, they have a big problem with that. Maybe they would, in fact, be better off if they defend themselves from North Korea."
When asked to clarify if he meant nuclear weapons, Trump responded: "Including with nukes, yes, including with nukes."
Trump later said he never said Japan should get nuclear weapons.
In February 2017, Trump stressed he intended for the US nuclear arsenal to stay "top of the pack".
He added: “I am the first one that would like to see … nobody have nukes, but we’re never going to fall behind any country even if it’s a friendly country, we’re never going to fall behind on nuclear power."
In the event of nuclear war, the US has large fall-out shelters which are being dusted off for the first time since the fall of the Soviet Union.
As tensions escalated over the summer with North Korea and Kim Jong-un, Trump famously warned he would retaliate with "fire and fury" if North Korea continued to threaten the US.
In October 2017, Trump sent nuclear bombers to fly towards North Korea in a dramatic show of force.