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.
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.