DEFCON is the "defense readiness condition," which is state of alertness to military conflict as determined from time to time by the United States Armed Forces.

There are five levels; the lower the number, the higher the state of alertness:

* DEFCON 5 -- normal readiness, this is the lowest state
* DEFCON 4 -- above normal state of readiness, intelligence watch and security measures are increased
* DEFCON 3 -- an increase in readiness above normal, the Air Force is ready to mobilize in 15 minutes if required
* DEFCON 2 -- next step to nuclear war, the Armed Forces are ready to deploy and engage in six hours
* DEFCON 1 -- maximum readiness/immediate response, nuclear war is imminent or has already begun

There is no worldwide or country-specific DEFCON status: it can and has been declared to include only specific geographical areas. Worldwide the level has never been declared above DEFCON 3.

DEFCON 1 has never been declared in United States history.

DEFCON 3 has been declared only three times: after the Yom Kippur War, in South Korea in 1976 after an axe murder in the Korean DMZ, and after the September 11 attacks. DEFCON 2 has been declared only twice: during the Cuban Missile Crisis and the opening phase of the Persian Gulf War.