If a nuclear missile is launched at the United States the President has just 6 minutes to come to terms with that and decide to launch a counter attack or not.

US nuclear deterrence in 2024 doesn’t rely on launch-on-warning, but on the expectation that no hostile power has the ability to locate and destroy the US ballistic missile submarine fleet prior to them performing their counterlaunches.

https://en.wikipedia.org/wiki/Second_strike

In nuclear strategy, a retaliatory strike or second-strike capability is a country’s assured ability to respond to a nuclear attack with powerful nuclear retaliation against the attacker. To have such an ability (and to convince an opponent of its viability) is considered vital in nuclear deterrence, as otherwise the other side might attempt to try to win a nuclear war in one massive first strike against its opponent’s own nuclear forces.

Submarine-launched ballistic missiles are the traditional, but very expensive, method of providing a second strike capability, though they need to be supported by a reliable method of identifying who the attacker is.

https://en.wikipedia.org/wiki/Launch_on_warning

Launch on warning (LOW), or fire on warning, is a strategy of nuclear weapon retaliation where a retaliatory strike is launched upon warning of enemy nuclear attack and while its missiles are still in the air, before detonation occurs.

In 1997, a US official stated that the US had the technical capability for launch on warning but did not intend to use a launch on warning posture and that the position had not changed in the 1997 presidential decision directive on nuclear weapon doctrine.

This non-reliance on launch-on-warning is also true of the French and British nuclear deterrents – the British don’t even maintain a nuclear arsenal other than on subs, so they haven’t even bothered with maintaining the option to do so, and the French only use tactical ALCMs in addition to the strategic sub-launched weapons; those weapons probably would be poorly-suited for such a role.

The Brits rather famously have the “letter of last resort”.

https://en.wikipedia.org/wiki/Letters_of_last_resort

The letters of last resort are four identically worded handwritten letters from the prime minister of the United Kingdom to the commanding officers of the four British ballistic missile submarines and stored on board of each. They contain orders on what action to take if an enemy nuclear strike has both destroyed the British government and has also killed or otherwise incapacitated both the prime minister and their designated “second person” of responsibility, typically a high-ranking member of the Cabinet such as the deputy prime minister or the first secretary of state. If the orders are carried out, the action taken could be the last official act of His Majesty’s Government.

If the letters are not used during the term of the prime minister who wrote them, they are destroyed unopened after that person leaves office, so that their content remains unknown to anyone except the issuer.

Process

A new prime minister writes a set of letters immediately after taking office and being told by the Chief of the Defence Staff “precisely what damage a Trident missile could cause”. The documents are then delivered to the submarines in sealed envelopes, and the previous prime minister’s letters are destroyed without being opened.

In the event of the deaths of both the prime minister and the designated alternative decision-maker as a result of a nuclear strike, the commander(s) of any nuclear submarine(s) on patrol at the time would use a series of checks to ascertain whether the letters of last resort must be opened.

According to Peter Hennessy’s book The Secret State: Whitehall and the Cold War, the process by which a Vanguard-class submarine commander would determine if the British government continues to function includes, amongst other checks, establishing whether BBC Radio 4 continues broadcasting.

In 1983, the procedure for Polaris submarines was to open the envelopes if there was an evident nuclear attack, or if all UK naval broadcasts had ceased for four hours.

Options

While the contents of these letters are secret, according to the December 2008 BBC Radio 4 documentary The Human Button, there were four known options given to the prime minister to include in the letters. The prime minister might instruct the submarine commander to:

• retaliate with nuclear weapons;

• not retaliate;

• use their own judgement; or,

• place the submarine under an allied country’s command, if possible. The documentary mentions Australia and the United States.

The Guardian reported in 2016 that the options are said to include: “Put yourself under the command of the United States, if it is still there”, “Go to Australia”, “Retaliate”, or “Use your own judgement”. The actual option chosen remains known only to the writer of the letter.