Donald Trump poised to take control of US nuclear weapons arsenal
When Donald Trump becomes the 45th president of the United States later on Friday he will be responsible for the word's second largest arsenal of nuclear weapons.
Mr Trump will have access to the "nuclear football" containing the codes allowing him to order a nuclear attack at a moment's notice.
Dr Trevor McCrisken, associate professor of US foreign policy at the University of Warwick, said Mr Trump would by now have received national security briefings.
Those are likely to have included a discussion about America's nuclear strategy and capabilities and the various scenarios that could occur, from ordering a limited strike or an all-out retaliation against an attack.
Dr McCrisken said: "What he will gain access to, once he has taken the oath of office, is the nuclear codes - basically a card which has a series of letters on it. He will know which of those amount to that day's codes."
The codes, known as "the biscuit", are thought to be updated every day.
They are carried by the president or contained in a briefcase - the so-called "nuclear football", held by an aide and kept close to the president at all times so he can be in direct contact with strategic command and act within minutes.
Dr McCrisken said: "He will always be within reach of those codes as if he suddenly gets told an attack has been launched, he has got a very limited amount of time to make a decision about what to do, to gain advice from his generals about what the correct response should be and to decide whether to initiate a strike."
The codes themselves simply confirm the president's identity as the individual giving the order. In the unlikely event that they were lost or a president was kidnapped, they would be useless to anyone else.
The nuclear football has a "black book", containing all the possible pro-programmed strike patterns the president could authorise, from a small, limited strike to a massive launch.
Even at the silos where the missiles are launched there is a different set of codes so those receiving the orders know they are genuine.
Dr McCrisken, who is also chairman of the security think tank the British-American Security Information Council, said: "Once the president has given the order to launch then the chain of command should follow.
"It all has to happen very quickly because the most likely scenario in which a president would order a nuclear attack would be if one had been launched against the United States."
Inter-continental ballistic missiles fired from Russia would leave the president no more than half an hour to make a decision, an attack from a submarine near the US considerably less.
In such an event, his national security adviser and possibly the secretary of defence and senior generals would suggest a response.
But any decision to respond lies with the president alone - as does the decision to strike first.
Dr McCrisken said: "It is expected that a president would be very responsible and will not use a first strike, as they call it, and will only use nuclear weapons if there is an attack upon the United States.
"However, that's not an agreed-upon policy that the president has entered into. Any president has it within his power to launch a first strike pretty much on his own personal whim.
"If he tried to do that, would his commanders follow the orders? He is not directly pressing a button - he enters a code and sends the orders.
"You could imagine perhaps a situation where he just loses his mind or gets angry and decides to launch.
"It's possible that his orders would be resisted or rejected, either at the command centre or maybe some of the silo operators would refuse to enter the codes they need to actually launch.
"But given how responsive people within a chain of command are to orders that come to them, it's not very likely. If your president tells you to do something it's very difficult to refuse that order.
"At the end of the day it's the president's conscience that you have to rely on."