Michael from Muckrock sez, "Starting on April 19, 1956, the federal government practiced and planned for a near-doomsday scenario known as Plan C. When activated, Plan C would have brought the United States under martial law, rounded up over ten thousand individuals connected to 'subversive' organizations, implemented a censorship board, and prepared the country for life after nuclear attack.
There was no Plan A or B."
Details of this program were distributed to each FBI field office. Over the following months and years, Plan C would be adjusted as drills and meetings found holes in the defensive strategy: Communications were more closely held, authority was apparently more dispersed, and certain segments of the government, such as the U.S. Attorneys, had trouble actually delineating who was responsible for what.
Bureau employees were encouraged to prepare their families for the worst, but had to keep secret the more in-depth plans for what the government would do if war did break out. Families were given a phone number and city for where the relocated agency locations would be, but not the exact location.
Plan C: The top secret Cold War countermeasure which would have brought the United States under martial law [Muckrock]