[edanm]

Look into the "Nuclear Triad". The US has three ways to fight back against a nuclear attack - land-based ICBMs, bombers, and submarines.

It is unknown how the US would react to a missile launched from NK, but for various reasons, it is likely to respond with a massive counter-attack. This is partially for deterrence (the whole idea behind the MAD doctrine), and partially for fear that if ICBMs are not launched right away, they could be deactivated by an incoming first strike.

Indeed, in the scenario laid out in the book (Spoiler Warning), after launching an ICBM, NK also launches missiles from a submarine that take out various parts of the US in minutes, far faster than the ~25 minutes for the ICBMs to get to the US from NK. Had the ICBMs not been launched before, it is probable they would be destroyed, taking out one leg of the "triad".