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 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).

Descriptions of DEFCONs

Defcons are phased increases in combat readiness. Expanded explanations follow below.

DEFCON 1: Maximum force readiness.

DEFCON 2: Increased force readiness (less than maximum).

DEFCON 3: Increased force readiness.

DEFCON 4: Peacetime; Increased intelligence; Strengthened security measures.

DEFCON 5: Peacetime.

 

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. The level can be increased, respectively, to DEFCON 4, 3, 2, and 1.

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.

DEFCON 3 refers to an increase in 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, 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.

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.

 

Conditions

Different branches of the armed forces (e.g., army, navy, air force) and different bases or command groups can be activated at different defense conditions. For much of the Cold War, US ICBM sites were always at DEFCON 4 rather than 5.

The highest alert condition the US military has been at was DEFCON 2. During the Cuban Missile Crisis, Strategic Air Command (SAC) was ordered to 2 and the rest of the US military to 3 on October 22, 1962. SAC remained at 2 until November 15. Higher alert conditions were also ordered during the Yom Kippur War (1973).

It should be further noted that SAC's move to DEFCON 2 during the Cuban Missile Crisis was precipitated by General Thomas Power and General Curtis LeMay, and was done without consulting the Executive Branch.

The DEFCON level is controlled primarily by the President and the Joint Chiefs of Staff, and each DEFCON level defines specific security, activation and response scenarios for the troops in question.

Some declassified documents suggest that DEFCON level 1 was ordered and attained during the Gulf War of 1991.