Reminder that the nuclear ballistic missile submarines were built for - and continue to be operated for - a war of total annhiliation. Assuming they were submerged when the first strike missiles launched, there would be a period of under an hour before they struck their targets. During that time, the ground based second strike might be launched by the United States, but France and the UK have no ground based nuclear launch capability. Operational depth is usually below radio depth, so there would be no contact until surfacing again, at which point the submarine commanders would have to face the knowledge that most people they ever knew were probably dead and their cities destroyed. They would then have to open their sealed orders and decide if they wanted to carry out another mass murder of civilians as a reprisal, find someone to surrender to, or simply turn the gun on themselves.