DFECON

Defense Condition is a measure of the activation and readiness level of the United States armed forces. DEF CON is the portmanteau of "DEFense [Readiness] CONdition". These defense conditions describe progressive postures for use between the Joint Chiefs of Staff and the commanders of unified commands. DEFCONs are matched to the situations of military severity. Standard peacetime protocol is DEFCON 5, which descends when plunged into increasingly severe situations. DEFCON 1 represents expectation of actual imminent attack, and has never been declared.

In a national state of emergency, seven different alert conditions (referred to as LERTCONs) can be issued. The seven LERTCONs are broken down into 5 Defense Conditions (referred to as DEFCONs) and two Emergency Conditions (referred to as EMERGCONs).

DEFCON 5 is the condition used to designate normal peacetime military readiness. An upgrade in military preparedness is typically made by the Joint Chiefs of Staff and announced by the United States Secretary of Defense.
DEFCON 4 refers to normal, increased intelligence and the heightening of national security measures. Readiness remained at this level throughout most of the Cold War. Ground combat authorized.
DEFCON 3 refers to an increase to force readiness above normal. Radio callsigns used by American forces change to currently-classifed callsigns. United States military commands (except for the Strategic Air Command, which was already at heightened alert on DEFCON 2) went to this level in 1962 during the Cuban Missile Crisis. All United States forces went on DEFCON 3 during the 1973 Yom Kippur War when the Soviet Union threatened to intervene on behalf of Egypt. The third time the United States reached DEFCON 3 was during the September 11, 2001 attacks. Naval and Air combat authorized.
DEFCON 2 refers to a further increase in force readiness just below maximum readiness. It has only been declared once, during the Cuban Missile Crisis.
DEFCON 1 refers to maximum readiness. This has never (verifiably) been used but is reserved for imminent or ongoing attack on US military forces or US territory by a foreign military power. Launch of nuclear missiles has been authorized.