"A new planning concept of area vulnerability, the 'Aiming Area Concept', was developed by FCDA during 1957. It is a more realistic basis for planning nonmilitary defensive measures in line with the destructive power of nuclear weapons. It recognizes the fact that many geographical areas in the United States contain multiple potential targets. Advisory Bulletin 214 defined an 'aiming area' as a geographic area in which all enemy would probably place one or more nuclear weapons to assure the destruction of the target. The bulletin described the concept, specified the method of determining an assumed aiming area, and informed States and local agencies how to assure reasonable accuracy and currency of aiming area. Advisory Bulletin 215 stated FCD A policy on using aiming area assumptions in civil defense programs. A command post exercise involving officials from all levels of government was held at FCDA Headquarters in March 1957. The exercise, Operation Sentinel, tested staff procedures and considered major operational problems which could result from a nuclear war against this country . The exercise emphasized the contribution which can be made to total nonmilitary defense by all agencies and levels of government. It informed the participants on the general circumstances of an attack, and provided experience in integrating representatives of other Federal agencies into a general staff activity. The success of the exercise resulted in scheduling additional similar exercises."
United States Emergency Management Institute: http://training.fema.gov/EMIWeb/edu/