DEFCON Levels History & List Of Alert Level ChangesHistory of official changes to the DEFCON level since the U.S started using the defense condition warning system. After NORAD's creation in 1957 the first DEFCON warning alert system in history was introduced. In the beginning the defense condition system was more complex and had 3 readiness levels: normal, increased and maximum and were divided into 8 alert conditions that were later simplified.
DEFCON 3: September 11th, 2001: Attack On The Twin Towers
DEFCON 2: January 15th, 1991: The Gulf War
DEFCON 3: August 18th, 1976: Operation Paul Bunyan
DEFCON 3: October 6th, 1973: Yom Kippur War
DEFCON 2: October 22nd, 1962: Cuban Missile Crisis