The United States has four threat levels above “normal” for military installations. Wright-Patterson Air Force Base and other bases around the country went from Threatcon Alpha, the lowest level, to Bravo, the next highest level. Here’s an explanation of each level:
THREATCON ALPHA: (Threat level low) This condition applies when there is a general threat of possible terrorist activity against personnel and facilities, the nature and extent of which are unpredictable, and circumstances do not justify full implementation of THREATCON BRAVO measures. However, it may be necessary to implement certain measures from higher THREATCONS resulting from intelligence received or as a deterrent. The measures in this THREATCON must be capable of being maintained indefinitely.
THREATCON BRAVO: (Threat level medium) This condition applies when an increased and more predictable threat of terrorist activity exists. The measures in this THREATCON must be capable of being maintained for weeks without causing undue hardship, affecting operational capability, and aggravating relations with local authorities.
THREATCON CHARLIE: (Threat level high) This condition applies when an incident occurs or intelligence is received indicating some form of terrorist action against personnel and facilities is imminent. Implementation of measures in this THREATCON for more than a short period probably will create hardship and affect the peacetime activities of the unit and its personnel.
THREATCON DELTA: (Threat level critical) This condition applies in the immediate area where a terrorist attack has occurred or when intelligence has been received that terrorist action against a specific location or person is likely. Normally, this THREATCON is declared as a localized condition. See also antiterrorism.