The Constitution is silent on what would happen if the President, Vice President, and most of Congress were suddenly annihilated in a nuclear or terrorist attack.
The Cold War, with the introduction of ballistic missiles carrying atomic weapons, made sudden "decapitation" of the government a very real prospect.
Responding to the growing Soviet nuclear threat, President Truman issued executive order 10346 in April 17, 1952. In it, he directed federal agencies to develop plans to continue their essential operations in case of nuclear war or other catastrophic event. It was in this order that the phrase "continuity of government" (COG) was first used, and it served as the foundation of several COG projects during the 1950s. Several of these were classified, including the construction of shelters and relocation facilities for the heads of various federal agencies and key staff. Most of these were located near Washington and based on the assumption that there would be sufficient advance warning to evacuate the president, vice president, key federal officials, and most members of Congress before the bombs arrived.
President Kennedy issued a flurry of executive orders in 1961. For example, executive order 10952 called for the Director of the Office of Emergency Planning to "develop plans, conduct programs and coordinate preparation for the continuity of governmental operations in the event of attack." Several additional executive orders by Kennedy directed the heads of various government agencies to develop contingency plans for dealing with an attack or other national emergency. It is also believed that construction of emergency command facilities and relocation facilities for government VIPs was greatly accelerated by Kennedy.
What exactly are the "essential functions" of the federal government that various COG programs are supposed to preserve? We don't know; most are classified. But the executive orders issued by Kennedy and his successors call for the heads of varipus agencies to develop contingency plans for relocation of the civilian population, seizure and control of all means of transportation (including private cars), food rationing, seizure and control of all radio and TV stations, and assigning civilians to work in jobs deemed "essential" (in other words, a civilian labor draft). The emergency COG plans would be put into action by Presidential executive orders.
INSIDE THE SHADOW GOVERNMENT: National Security and the Cult of Secrecy reproduces the full text of the executive orders issued by Kennedy and other Presidents relating to COG programs.