It’s hard to overstate how thoroughly the U.S. military has prepared for doomsday — the day America gets into a nuclear shooting war.
No detail seems to have been overlooked. There’s even a designated “safe escape” door at the nuclear-warfighting headquarters near Omaha, Nebraska, through which the four-star commander would rush to a getaway plane moments before the first bomb hit.
Procedures are in place for ensuring U.S. nuclear weapons are ready for a presidential launch order in response to — or in anticipation of — a nuclear attack by North Korea or anyone else. There are backup procedures and backups for the backups. Source.