The DEFCON Alert System was devised by The US Military in 1966, during The Cold War Era. The System was born out NORAD (North American Aerospace Defense Command), which had been tasked as was SAC (The US Air Force's Strategic Air Command) – to monitor potential Soviet Russian activities which may lead to a Nuclear War (as well as fight one). As such a standardized notification System was needed to Alert US Military Assets. Hence DEFCON was born. The System (also slightly modified) is still used by The US Military even today.
While often seen (in Film and Novel), DEFCON is often associated with color identifiers. This is somewhat fictional, in that The Alert Stages do not need a specific color. Much of The System is tied into the various stages advanced to during potential Conflicts. While developed for Nuclear War scenarios, DEFCON is also (currently) used to Alert Military Assets to other Political and Military situations (destabilized Governments, Terrorism, etc.)
Originally, DEFCON was only (4) Levels. In the 1990s, a 5th Level was adopted. This Level is now considered the lowest stage of Readiness. It is known by The Code ''FADE OUT”. The 4th Level is known by The Code ''DOUBLE TAKE''. This Stage is a somewhat increased state of Alert, often involving Space and Air Surveillance (as well as ground Assets) – against a potential threat.
The 3rd Level, DEFCON 3 (Code ''ROUND HOUSE'') is a much higher and more dangerous Alert Stage. At this point US Military Assets are alerted to a 15-minute full deployment level. Following, is 2nd Level, DEFCON 2 (Code ''FAST PACE''). The US has only come to this Level (officially), once (The Cuban Missile Crisis of 1962). This Stage is the primary step to Nuclear War, with US Nuclear Assets no less than 6-hours from deployment.
Finally, The 1st Level, DEFCON 1 (Code ''COCKED PISTOL''), has never been deployed. This is a Full deployment of American Nuclear Assets (Land, Air and Sea). This can be viewed as a First or Retaliatory Strike.