Able Archer 83 was a military exercise conducted by NATO that took place in November 1983, as part of the annual Able Archer 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. military attaining a simulated DEFCON 1 coordinated nuclear attack. The five-day exercise, which involved NATO commands throughout Western Europe, was coordinated from the Supreme Headquarters Allied Powers Europe (SHAPE) headquarters in Casteau, Belgium.

The 1983 exercise, which began on November 7, 1983, introduced several new elements not seen in previous years, including a new, unique format of coded communication, radio silences, and the participation of heads of government. This increase in realism, combined with tense 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 and military to believe that Able Archer 83 was a ruse of war, obscuring preparations for a genuine nuclear first strike. In response, the Soviet Union readied their nuclear forces and placed air units in East Germany and Poland on alert. The Soviet 4th Air Army began loading nuclear warheads onto combat planes in preparation for war. The apparent threat of nuclear war ended when U.S. Lieutenant General Leonard H. Perroots advised against responding to the Warsaw Pact military activity, which ended with the conclusion of the exercise on November 11.

The exercise attracted public attention in 2015 when the President's Intelligence Advisory Board's 1990 report on the exercise was declassified. Some scholars have argued that Able Archer 83 was one of the times when the world has come closest to nuclear war since the Cuban Missile Crisis in 1962. The declassification of related documents in 2021 supported this notion. Other scholars have disputed this.

The greatest catalyst to the Able Archer war scare occurred more than two years earlier. In a May 1981 closed-session meeting of senior KGB officers and Soviet leaders, General Secretary Leonid Brezhnev and KGB chairman Yuri Andropov bluntly announced that the United States was preparing a secret nuclear attack on the USSR.

To combat this threat, Andropov announced, the KGB and GRU military foreign intelligence arm would begin Operation RYaN. RYaN (Russian: РЯН) was a Russian acronym for 'nuclear missile attack' (Ракетно Ядерное Нападение, Raketno Yadernoe Napadenie); Operation RYaN was the largest, most comprehensive peacetime intelligence-gathering operation in Soviet history. Agents abroad were charged with monitoring the figures who would decide to launch a nuclear attack, the service and technical personnel who would implement the attack, and the facilities from which the attack would originate. It is possible that the goal of Operation RYaN was to discover the first intent of a nuclear attack and then preempt it.

The exact impetus for the implementation of Operation RYaN is not known for sure. Oleg Gordievsky, the highest-ranking KGB official ever to defect, attributed it to "a potentially lethal combination of Reaganite rhetoric and Soviet paranoia". Gordievsky conjectured that Brezhnev and Andropov, who "were very, very old-fashioned and easily influenced ... by Communist dogmas", truly believed that an antagonistic Ronald Reagan would push the nuclear button and relegate the Soviet Union to the literal "ash heap of history". Central Intelligence Agency historian Benjamin B. Fischer lists several concrete occurrences that likely led to the birth of RYaN. The first of these was the use of psychological operations (PSYOP) that began soon after Reagan took office.

In his report, Fischer also writes that another CIA source was, at least partially, corroborating Gordievsky's reporting. This Czechoslovak intelligence officer—who worked closely with the KGB on RYaN—"noted that his counterparts were obsessed with the historical parallel between 1941 and 1983. He believed this feeling was almost visceral, not intellectual, and deeply affected Soviet thinking."

Psychological operations by the United States began in mid-February 1981 and continued intermittently until 1983. These included a series of clandestine naval operations that stealthily accessed Soviet territorial waters in the far north and far east, demonstrating how close NATO ships could get to critical Soviet military bases. In 1981, a group of 83 American, British, Canadian, and Norwegian ships led by the USS Dwight D. Eisenhower sailed through the Greenland–Iceland–United Kingdom (GIUK) gap undetected by Soviet radar and spy satellites, reaching the Kola Peninsula. There were other operations routinely occurring in the Barents, Norwegian, Black, and Baltic seas. US intelligence ships were regularly posted off the coast of the Crimean Peninsula. American bombers also flew directly towards Soviet airspace, peeling off at the last moment, sometimes several times per week. These near-penetrations were designed to test Soviet radar vulnerability as well as demonstrate US capabilities in a nuclear war.