I can't imagine the military not having a formal procedure for something this important. Some secret equivalent to the 25th Amendment, but appropriate for quickly delegating warlord powers in an existential emergency.

Wonder what that chain would be? VP, skip the Speaker, Secretary of State, Defense, DHS...obvious beginning, but it would have to go deep to cover a big decapitation strike, like Tom Clancy envisioned.

If I had to pick an existing system to call the "Doomsday Weapon", I'd nominate nuclear-powered & -armed submarines. Mostly autonomous, and just because "normally" they wouldn't launch without orders from the chain of command, they also by nature have to be capable of launching on their own, with no more authority than two officers turning their keys. Has to be that way, otherwise the system has a gaping vulnerability to jamming or some other means of breaking the tether. A boomer prioritizes issue #2 over #1. If two officers go nuts, they can launch.

Once upon a time (two years ago), that would have seemed scary. But now we live with the knowledge that one single guy has the launch codes, and he's already nuts.