If Trump decides he has had enough of Kim Jong-un’s verbal threats, he merely has to turn to the low-level military aide at his side and ask them to open up the black briefcase that officer keeps permanently in their grasp. The bag is known as the nuclear “football”. (It gets its name from the code word for the very first set of nuclear war plans: dropkick.) Inside the bag is a menu of options, explained in detail in a “black book,” but also set out in a single, cartoon-like page for speedy comprehension. Trump has only to make his choice, pick up the phone to the Pentagon war room, utter the code words that identify him as the president and give the order. That’s it.