Britain has a new Prime Minister, Boris Johnson. And on his first day of office — as is the case with all new prime ministers — he will have to handwrite four identical letters called the Letters of Last Resort. They are instructions to the captains of Britain’s four nuclear submarines, known as the Trident deterrent. The letters are entirely secret and entirely personal to each prime minister. That is, unless London gets destroyed by a nuclear blast and the British government is wiped out. If that happens, the letters will be unsealed, and the submarine commanders will follow the orders given to them from the British prime minister from beyond the grave, written on that fateful first day in office.

The question we tackle today: what should you write in that situation? Should you retaliate, knowing that it could escalate toward nuclear apocalypse? Do the letters most likely say to retaliate? Nuclear deterrence requires a commitment to do the unthinkable: to be ready to use nuclear weapons if attacked. But could you live with yourself if it was actually you giving the order?

We speak to Lord Butler, who advised five prime ministers on how to write the Letters of Last Resort; former US Secretary of Defense William Perry under Bill Clinton; Oxford philosophy professor Jeff McMahan; and Cristina Varriale of RUSI.