In November 1983, NATO's largest military exercise of the Cold War era, Able Archer 83, took place.
During Able Archer 83, NATO forces practiced sequentially moving troops into combat readiness levels from DEFCON 5 (peacetime) to DEFCON 1 (war). Since each level of combat readiness was practiced sequentially, KGB informants perceived them as a real combat alert. According to intelligence information, NATO military doctrine stated: "Operational readiness No. 1 is declared when there are obvious prerequisites for conducting a military operation. When it is clearly established that war is inevitable and can begin at any moment."
According to Western historians, the leadership of the Soviet Union believed that the only chance to withstand a NATO attack was to get ahead of it. In this regard, the CIA noted increased activity in the Baltic military district, the Czechoslovak Socialist Republic, and in the locations of nuclear-armed aircraft carriers in the Polish People's Republic and the GDR: "all troops were put on alert, nuclear weapons storage facilities were opened."
The scenario published by NATO details a hypothetical scenario for Exercise Able Archer, which was used by the US Joint Chiefs of Staff in Washington, DC, and the UK Ministry of Defence in London. The war game would have involved Force Blue, representing NATO, and Force Orange, representing the Warsaw Pact. The scenario involved proxy conflicts escalating in Syria, South Yemen and Iran after Yugoslavia defected to the Blue bloc and Orange invaded Finland, Norway and West Germany.
Even modern historians admit that 1983 was much closer to a nuclear war than even the Cuban Missile Crisis of 1962. However, they officially deny the fact of a conflict between NATO and Warsaw Pact countries, which happened in November 1983.