Defcon Level October 22nd, 1962: Cuban Missile Crisis
DEFCON 2 was reached on October 22nd, 1962 during the Cuban missile crisis. The United States Armed forces were raised to DEFCON level 3 and the Strategic Air Command were raised to DEFCON 2 until the 15th of November, 1962. Department of State: Cuban Missile Crisis.
This was the first time in history that the alert level was changed to lower than 5. Only the Strategic Air Command (SAC) reached alert level 2 however, while the United States armed forces was ordered to alert level 3.