Plan C (240kb PDF) was a plan developed by the United States in the mid-1950s to impose martial law in the event of major national disaster, such as a nuclear war.
Part of that plan included an official enemies list of people who would be rounded up,
II. Emergency Programs
(a) Emergency Detention Program–The Federal Bureau of Investigation schedules for apprehension individuals whose affiliations with subversive organizations are so pronounced that their continued liberty in the event of a national emergency would present a serious threat to the internal security of the country. As of April 17, 1956, 12,949 individuals were scheduled for apprehension in an emergency.
Apprehensions under this program would be instituted upon instructions from the Attorney General after the declaration of an emergency.
Detailed plans for institution of this program have been drawn up and are in the possession of the various field offices of this Bureau. In addition, all necessary forms to be used in conjunction therewith have been made available to all field offices.
It would be interesting to see who exactly was on that list of people to be rounded up.