Entering office as Britain's new prime minister on Wednesday, Boris Johnson will write the toughest letter of his life: providing orders of last resort to Britain's nuclear ballistic missile submarine commanders.

Those letters provide orders to Britain's four Vanguard-class submarine commanders on how to proceed if the British government is decapitated in a nuclear strike.

Recognizing that it isn't exactly an easy task for any new prime minister to write such a letter, senior government officials will give Boris Johnson some guidance. This will center on advice from the Cabinet Secretary and National Security Adviser Sir Mark Sedwill, the most senior uniformed officer of the British armed forces Sir Nick Carter and Royal Navy Head Admiral Tony Radakin.

The options include orders to retaliate against previously selected targets. In the event of a Russian nuclear strike on London, for example, one option would be launching a counterstrike against Moscow. Reports suggest the prime minister can also order no retaliation, order a commander to use their own discretion, or to seek orders from an allied power.

At the top of the list of allied powers is the United States, for two reasons.

First, because British nuclear strike planning assumes the U.S. will also be at war with whoever has just nuked Britain. Second, because U.S. and British submarine crews are intimately familiar with each others' capabilities and strategies. Putting British submarine crews under U.S. authority would thus maximize their prospective utility to the joint war effort.

Still, the letters aren't Johnson's only task.

Johnson must also appoint a number of "Nuclear Deputies" from his new cabinet. Their role defined under a highly classified "Nuclear Release Procedures" guide, these deputies are responsible for taking command of British nuclear forces in the event the prime minister is assassinated. The identity of those deputies a prime minister selects is never identified so as to mitigate the risk that they might be targeted for assassination in preemption of any nuclear attack on Britain.

What will Johnson put in his letters? We'll likely never know. But Johnson's predecessor wisely made clear she would use nuclear weapons in Britain's defense.