Able Archer 83 was a military exercise conducted by NATO that took place in November 1983, as part of an annual exercise. It simulated a period of heightened nuclear tensions between NATO and the Warsaw Pact, leading to concerns that it could have been mistaken for a real attack by the Soviet Union. The exercise is considered by some to be one of the closest moments the world came to nuclear war during the Cold War. The purpose of the exercise, like previous years, was to simulate a period of conflict escalation, culminating in the U.S.
https://en.wikipedia.org/wiki/Able_Archer_83