[arethuza]

The Letters of Last Resort are real:

https://en.wikipedia.org/wiki/Letters_of_last_resort

One important part of the UK's nuclear strategy is that the UK Trident warheads don't have PALs - the crews have everything they need to launch. The reasoning behind this being that, in the dark days of the Cold War, it was expected that there wouldn't be enough time to transmit an authorization message in the time between a nuclear strike being detected and the weapons detonating so any retaliation would be based purely on the judgement of the crews on the subs:

https://en.wikipedia.org/wiki/Four-minute_warning

[pja]

It’s well attested that the people in charge of the Minuteman missle systems had the same concerns & set the PAL codes on all the missiles to 000000.