U.S. Had Plans for “Full Nuclear Response” In Event President Killed or Disappeared during an Attack on the United States.

From the Nukevault

Newly declassified document expands limited public record on nuclear “pre-delegation”.

Both USSR and China were to be targeted simultaneously, even if attack were conventional or accidental, and regardless of who was responsible.

LBJ ordered a change in instructions in 1968 to permit more limited response, avert “dangerous” situation.

See: Electronic Briefing Book No. 406