In the event of a national emergency, a series of seven different alert Conditions (LERTCONs) can be called. The 7 LERTCONs are broken down into 5 Defense Conditions (DEFCONs) and 2 Emergency Conditions (EMERGCONs).
Defense readiness conditions (DEFCONs) describe progressive alert postures primarily for use between the Joint Chiefs of Staff and the commanders of unified commands. DEFCONs are graduated to match situations of varying military severity, and are numbered 5,4,3,2, and 1 as appropriate. DEFCONs are phased increases in combat readiness. In general terms, these are descriptions of DEFCONs:
DEFCON 5 Normal peacetime readiness
DEFCON 4 Normal, increased intelligence and strengthened security measures
DEFCON 3 Increase in force readiness above normal readiness
DEFCON 2 Further Increase in force readiness, but less than maximum readiness
DEFCON 1 Maximum force readiness.
EMERGCONs are national level reactions in response to ICBM (missiles in the air) attack. By definition, other forces go to DEFCON 1 during an EMERGCON.
DEFENSE EMERGENCY: Major attack upon U.S. forces overseas, or allied forces in any area, and is confirmed either by the commander of a unified or specified command or higher authority or an overt attack of any type is made upon the United States and is confirmed by the commander of a unified or specified command or higher authority.
AIR DEFENSE EMERGENCY: Air defense emergency is an emergency condition, declared by the Commander in Chief, North American Aerospace Defense Command. It indicates that attack upon the continental United States, Canada, or US installations in Greenland by hostile aircraft or missiles is considered probable, is imminent, or is taking place.
During the Cuban Missile Crisis, the US Strategic Air Command was placed on DEFCON 2 for the first time in history, while the rest of US military commands (with the exception of the US Air Forces in Europe) went on DEFCON 3. On 22 October 1962 SAC responded by establishing Defense Condition Three (DEFCON III), and ordered B-52s on airborne alert. Tension grew and the next day SAC declared DEFCON II, a heightened state of alert, ready to strike targets within the Soviet Union.
On 15 November 1965 the day Strategic Air Command (SAC) postured down to defense condition (DEFCON) III.
On 6 October 1973 Egyptian and Syrian forces launched a surprise attack on Israel. On 25 October U.S. forces went on Defense Condition (DEFCON) III alert status, as possible intervention by the Soviet Union was feared. On 26 October, CINCSAC and CINCONAD reverted to normal DEFCON status. On 31 October USEUCOM (less the Sixth Fleet) went off DEFCON III status. The Sixth Fleet resumed its normal DEFCON status on 17 November 1973.