The defense readiness condition (DEFCON) is an alert state used by the United States Armed Forces. The DEFCON system was developed by the Joint Chiefs of Staff and unified and specified combatant commands. It prescribes five graduated levels of readiness (or states of alert) for the U.S. military, and increase in severity from DEFCON (least severe) to DEFCON 1 (most severe) to match varying military situations.

DEFCONs are a subsystem of a series of Alert Conditions, or LERTCONs, that also includes Emergency Conditions (EMERGCONs). There is no single DEFCON status for the country, and in fact different branches of the military can be at different levels of DEFCON at the same time. DEFCONs should not be confused with similar systems used by the U.S. military, such as Force Protection Conditions (FPCONS), Readiness Conditions (REDCONS), Information Operations Condition (INFOCON) and its future replacement Cyber Operations Condition (CYBERCON), and Watch Conditions (WATCHCONS), or the former Homeland Security Advisory System used by the United States Department of Homeland Security.

Levels

DEFCONs vary between many commands and have changed over time, and the United States Department of Defense uses exercise terms when referring to the DEFCONs. This is to preclude the possibility of confusing exercise commands with actual operational commands. On 12 January 1960, NORAD "proposed the adoption of the readiness conditions of the JCS system", and information about the levels was declassified in 2006:

Movies and popular culture often misuse the DEFCON system by "going to DEFCON 5" when a nuclear war is imminent. In fact, DEFCON 5 is the lowest state of readiness. The highest state, DEFCON 1, has never been called for.