The HMS Vengeance is shown returning to Naval Base Clyde. The Vengeance carries the Trident ballistic missile, Britain's nuclear deterrent. (Tam McDonald/Royal Navy)

After a tumultuous few weeks in British politics, Theresa May officially became Britain's prime minister on Wednesday.

The day was full of tradition and pomp. Her predecessor, David Cameron, traveled to Buckingham Palace to formally tender his resignation to Queen Elizabeth II and announce May as his successor. The queen then met May at the palace, asked her if she can form a government and then formally asked her to do so. May became the 13th prime minister to air-kiss the hand of the queen, who is 90 years old.

But aside from all the pageantry, there's some serious weight to the day. Not only will May have to form a functioning government and deal with Britain's eventual exit from the European Union — she will also soon have to write a "letter of last resort" for use in the event of a devastating nuclear attack on Britain.

This letter is a handwritten note to the commanders of Trident missile submarines on patrol. These four submarines carry nuclear-armed intercontinental ballistic missiles that could cause a devastating atomic explosion halfway across the world: Since 1998, they have been Britain's sole operating nuclear weapons system. At least one of these submarines is always on patrol in a secret location, ready to fire its weapons should something calamitous happen.

In theory, only Britain's prime minister or a second designated person (chosen by the prime minister but not publicly identified) can give the launch order. The letter of last resort is designed to give instructions if both are no longer alive or are completely out of contact. It is placed in a safe in each submarine, only to be opened if Britain's government is wiped out.

The letter details a number of different options for reactions in different scenarios. The Guardian reports that options are said to include things like "Retaliate" or "Put yourself under the command of the U.S., if it is still there." The truth is that extremely few people know exactly what is in these letters; there has never been a situation where they have had to be opened. The letters are destroyed after the prime minister in question leaves office and replaced by a new letter.

Robin Butler, a former cabinet secretary who had briefed two prime ministers on how to write the letter, explained to the BBC in 2010 just how "desperately" secret they were: "I mean they are clearly really secret because the whole point of a nuclear deterrent is ... your enemy doesn't know what he may incur if he attacks you, and so these are highly secret things, and only one person, who is the initiator of them, knows what the orders are, and that is the prime minister."

What is known is that May will be asked to write the letter as soon as she takes office. She will meet with the chief of the defense staff, who will explain, in detail, the damage such a nuclear strike could cause. It's obviously a tough moment for the new prime minister. May has to write an unusually intimate handwritten letter that assumes not only that she and many other Britons have already been killed but that also could condemn millions of other people to their own deaths.

Speaking to the Daily Mail in 2008, Charles Guthrie, former chief of the defense staff, explained that Tony Blair had become "quite quiet" when it came to making the decision. "I think quite honestly, like most prime ministers, he hadn’t given a huge amount of thought to what this really meant. And it is actually an awesome responsibility," Guthrie said. "It really comes home to you that he could, if the circumstances demanded it, create devastation on a huge scale."

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