Ooops (almost)

Wiki:

Able Archer 83 was a ten-day NATO command post exercise starting on November 2, 1983 that spanned Western Europe, centred on SHAPE's Headquarters situated at Casteau, north of the Belgian city of Mons. Able Archer exercises simulated a period of conflict escalation, culminating in a coordinated nuclear release. The 1983 exercise 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 1983 exercise, coupled with deteriorating relations between the United States and the Soviet Union and the anticipated arrival of Pershing II nuclear missiles in Europe, led some members of the Soviet Politburo to believe that Able Archer 83 was a ruse of war, obscuring preparations for 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 threat of nuclear war abruptly ended with the conclusion of the Able Archer 83 exercise on November 11.