Fascinating article from Wikipedia on a major Cold War event:
Able Archer 83 was a ten-day NATO exercise starting on November 2, 1983 that spanned the continent of Europe and simulated a coordinated nuclear release. It incorporated a new, unique format of coded communication, radio silences, participation by heads of state, and a simulated DEFCON 1 nuclear alert. The realistic nature of the exercise, coupled with deteriorating relations between the United States and the Soviet Union and the anticipated arrival of “super-stealth” Pershing II nuclear missiles in Europe, led some in the USSR to believe that Able Archer 83 was a genuine nuclear first strike. In response, the Soviets readied their nuclear forces and placed air units in East Germany and Poland on alert. This relatively obscure incident is considered by many historians to be the closest the world has come to nuclear war since the Cuban Missile Crisis of 1962. The immediate threat of nuclear war abruptly ended with the conclusion of the Able Archer 83 exercise on November 11, which, coincidentally, was also Remembrance Day.