Ron Rosenbaum in Slate:
In the case of the Letter of Last Resort, the reference turns out to be factual: At this very moment, miles beneath the surface of the ocean, there is a British nuclear submarine carrying powerful ICBMs (nuclear-armed intercontinental ballistic missiles). In the control room of the sub, the Daily Mail reports, “there is a safe attached to a control room floor. Inside that, there is an inner safe. And inside that sits a letter. It is addressed to the submarine commander and it is from the Prime Minister. In that letter, Gordon Brown conveys the most awesome decision of his political career … and none of us is ever likely to know what he decided.”
The decision? Whether or not to fire the sub’s missiles, capable of causing genocidal devastation in retaliation for an attack that would—should the safe and the letter need to be opened—have already visited nuclear destruction on Great Britain. The letter containing the prime minister’s posthumous decision (assuming he would have been vaporized by the initial attack on the homeland) is known as the Last Resort Letter.