the United States had a contingency plan in effect where, in the event that the President went missing or was killed during an attack on the country, the military was instructed to launch an automatic and simultaneous "full nuclear response" against both the Soviet Union and China. And it wasn't until 1968 that the government under Lyndon Johnson repealed the directive.
From the NSA's official statement:
Prior to President Johnson's decision, instructions for the emergency use of nuclear weapons that both he and his predecessors had previously approved stipulated a full-scale nuclear counter-attack even if the initial strike were conventional, or the result of an accident, and both Communist giants would be targeted regardless of whether either of them had launched the first strike.
This new information is contained in a record of a meeting between President Johnson and his top national security advisers on 14 October 1968. At the meeting, Johnson's military and civilian aides unanimously recommended that the standing orders, known by the code-name "Furtherance," be revised substantially in order to reduce the inherent risks involved. The changes included providing instructions to commanders to respond to a conventional attack with conventional weapons-an implicit "no-first use" nuclear policy. At the session, speaking of the new approach, National Security Advisor Walt Rostow advised Johnson: "We think it is an essential change. This was dangerous." The entire Joint Chiefs of Staff concurred.
"This was dangerous," they said. Jeez, d'ya think? Man, it's a good thing JFK didn't go philandering off somewhere during the Cuban Missile Crisis.