If you have a secret to keep, there's no better place to keep it than in a safe inside another safe in the control room of a nuclear-class submarine deployed deep in the ocean. It takes a truly momentous secret to warrant such treatment—a matter not just of life or death, but of potentially millions of lives or millions of deaths. There are four such safes inside other safes inside submarines, and each one contains an identical hand-written letter by Britain's Prime Minister with instructions on what to do in the event that the country is wiped out by nuclear attack.
The scenario: Britain has been obliterated by nukes. The Prime Minister is known to be dead, an unidentified second government official is dead (this is another secret that each Prime Minister is responsible for: establishing a person to be designated as his alternate nuclear decision-maker in case of his death). The commanders of the submarines with nuclear capabilities open the safes, break the seal on the letters and do as the Prime Minister posthumously commands. There is one of two possible directions inside: either retaliate—and kill millions of innocent, although foreign, civilians—or don't.
Making that weighty hypothetical decision is one of the first tasks performed by every new Prime Minister. If the letters go unused, as they always have, they are destroyed—unread—when the premiership changes hands. Truly unknowable secrets.
Some flaws in the system
A 2008 BBC documentary detailing Britain's nuclear protocol piqued the interest of Ron Rosenbaum, who has written about the potential for a nuclear World War III. He took to Slate to explain exactly why, "with all due respect to our British cousins, this seems, well, insane."
It is not simply the incongruousness of a quaint hand-written letter serving as the ultimate directive for nuclear use that drove Rosenbaum to call the process "profoundly shocking." There is the practical problem of how the sub commander would know whom to aim the missiles at if everyone up on the mainland had already been turned to ash. Effectively establishing with an sufficient level of certainty that everyone back on land had been killed in a nuclear attack seems problematic. Some sources claim the broadcasting, or not, of Radio 4 is the signal of nuclear genocide, but that seems reprehensibly glitchy.
The Paradox of MAD
The purpose of broadcasting a nation's nuclear capabilities is to reap the deterrent benefits of mutually assured destruction (MAD). Hopefully, if you know we could nuke you, it will discourage you from nuking us. But does retaliation in the case of an unstoppable nuclear attack even make moral sense? Introducing the letters into this equation doesn't change the ethics, but it does highlight the paradox. In a conversation with Rosenbaum on This American Life, Ira Glass sums up the problem detailed in the Slate article, saying, "Doesn't it undermine the whole point of having nuclear weapons in the first place to publicize the fact that there's a secret letter that very well might say no, don't retaliate?"
But of course, why would you retaliate? Rosenbaum says it best:
If your nation has been wiped out by a nuclear strike, is there any point in retaliating and killing tens, hundreds of millions of innocent people when the threat to retaliate has already failed?
So then, the doubt implied by a decision to be made is dangerous, but the certainty required for MAD is immoral. Thus, the benefit of the letters—and the benefit of making their existence known—is in allowing civilians to feel at ease with the Schrodinger's Cat situation. It's hard to top Glass' eloquence:
[T]hen the advantage of having this letter with the order locked away in a box inside another box—what it does is that you don't want your enemy to know that you're not going to launch those missiles, but you don't want to think for yourself that you are. So it lets you believe both things. It lets two truths exist at the same time.
So in the case of the Letters of Last Resort, it's not just the unknown information that's important—the act of secrecy itself serves a national purpose.