The “Letters of Last Resort” are a set of four identically-worded hand-written letters penned by the Prime Minister of the United Kingdom, composed shortly after they assume office.

These letters are of immense significance, as they contain orders to be followed by the commanders of Britain’s nuclear submarines in the event that the government is incapacitated by a nuclear attack.

Contents

Background

The Cold War was characterized by a persistent state of geopolitical tension and the threat of nuclear war. The doctrine of nuclear deterrence arose as a strategy to prevent actual nuclear conflict. The logic was that neither side would risk initiating a nuclear war knowing it would lead to mutual destruction. In this context, the UK, as a major ally of the US and a member of NATO, sought to maintain its own nuclear deterrent to contribute to this balance of power.

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The United Kingdom, having developed its own nuclear weapons independently in the 1950s, became one of the world’s recognized nuclear powers. The British government saw maintaining a nuclear arsenal as essential to its national security and international standing, particularly given the Soviet Union’s expanding nuclear capabilities.

The UK’s strategic approach included the development of a Continuous At-Sea Deterrence (CASD). This involved having at least one of its nuclear-armed submarines on patrol in the oceans at all times. These submarines, known as the Vanguard-class, were equipped with Trident ballistic missiles and were the cornerstone of the UK’s nuclear deterrent.

A Vanguard-Class submarine fires a Trident II missile.

In this high-stakes environment, the UK government had to confront the possibility of a decapitating nuclear strike that could obliterate its command and control structures, including the political leadership. The Letters of Last Resort were conceived as a solution to this existential dilemma. They ensured that, in the event of such a catastrophe, the commanders of the submarines on patrol would still have clear instructions on how to proceed, thus maintaining the credibility of the UK’s nuclear deterrent.

The Contents Of The Letters

The content of the Letters of Last Resort are shrouded in secrecy and speculation, as they are one of the most closely guarded secrets in the realm of national security. These letters, written by the Prime Minister of the United Kingdom, contain instructions for the commanders of the nation’s nuclear-armed submarines.

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They are only to be opened and acted upon if the submarine commanders believe that the British government has been incapacitated by a nuclear strike. The exact nature of these instructions has never been publicly disclosed, but it is generally understood that they outline several potential courses of action.

Retaliation: The most direct option would be for the submarine commanders to launch a retaliatory nuclear strike against the aggressor nation. This aligns with the principle of mutually assured destruction (MAD), a cornerstone of Cold War nuclear strategy.

Do Not Retaliate: This option instructs the commanders not to launch a counterstrike. This choice could be made for various reasons, such as the belief that further nuclear exchanges would be futile or morally indefensible, or that preserving what remains of humanity is a higher priority than retaliation.

Use Own Judgment: Here, the Prime Minister delegates the decision to the submarine commanders, trusting in their judgment to make the best decision based on the situation at hand.

Join Allies: In this scenario, the submarine commanders would be instructed to place themselves under the command of an allied nation, presuming that nation’s government is still functioning. This could be a NATO ally, for instance, the United States or Australia.

Change Of Leadership

The “Letters of Last Resort” are intrinsically linked to the procedure followed by the United Kingdom’s government whenever there is a change in the Prime Minister. This procedure underscores the continuity and gravity of the nation’s nuclear command and control responsibilities, irrespective of political changes or individual leadership styles.

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Upon assuming office, one of the first duties of the new Prime Minister is to write the Letters of Last Resort. This task is undertaken shortly after the Prime Minister is briefed on nuclear weapons deployment and operation.

HMS Vigilant is one of the four Vanguard-Class submarines that carries a letter.

Each Prime Minister writes these letters personally and in total secrecy. They receive no advice from aides or military officials on what to write.

While the exact content remains classified, it is understood that the Prime Minister provides explicit instructions on what action the Trident submarine commanders should take if they determine that the British state has been incapacitated by a nuclear attack.

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Once written, the letters are sealed and sent to the Royal Navy’s four Vanguard-class submarines. Each submarine carries a copy of the letter, stored in a safe, only to be opened if the worst-case scenario unfolds.

As a new Prime Minister writes their letters, the ones written by their predecessor are removed from the submarines and destroyed, unopened. This is a critical aspect of the procedure, ensuring that the instructions always reflect the current Prime Minister’s wishes and the potential change in policy or strategy that might come with new leadership.