During 10 days in November 1983, the United States and the Soviet Union nearly started a nuclear war. Newly declassified documents from the CIA, NSA, KGB, and senior officials in both countries reveal just how close we came to mutually assured destruction – over a military exercise.

That exercise, Able Archer 83, simulated the transition by NATO from a conventional war to a nuclear war, culminating in the simulated release of warheads against the Soviet Union. NATO changed its readiness condition during Able Archer to DEFCON 1, the highest level. The Soviets interpreted the simulation as a ruse to conceal a first strike and readied their nukes. At this period in history, and especially during the exercise, a single false alarm or miscalculation could have brought Armageddon.

According to a diplomatic memo obtained through a Freedom of Information Act request by National Security Archives researcher Nate Jones, Soviet General Secretary Yuri Adroprov warned U.S. ambassador Averell Harriman six months before the crisis that both countries "may be moving toward a red line" in which a miscalculation could spark a nuclear war. Harriman later wrote that he believed Andropov was concerned "over the state of U.S.-Soviet relations and his desire to see them at least 'normalized,' if not improved."

The early 1980s was a "crisis period, a pre-wartime period," said Gen. Varfolomei Korobushin, the former deputy chief of staff of the Soviet nuclear Strategic Rocket Forces, according to an interview conducted by the Pentagon in the early 1990s and obtained by Jones. The Kremlin's Central Committee slept in shifts. There were fears the deployment of Pershing II ballistic missiles to Europe (also in November 1983) could tip the balance. If a conventional war erupted, Soviet planners worried their troops would come close to capturing the nuclear-tipped missiles, prompting the United States to fire them.

The Soviet Union, according to an unclassified article written for the CIA's classified Studies in Intelligence journal and provided to Jones, notes that Soviet fears of a preemptive American nuclear attack "while exaggerated, were scarcely insane." This stemmed from the Soviet experience during World War II, when the Third Reich launched Operation Barbarossa, the largest invasion in human history. Soviet officials worried history might be repeated by NATO.

Oleg Gordievsky, a CIA and MI6 source during the Cold War, was previously known to have warned the West about these fears, but the CIA article identifies a second source of this information: a Czech intelligence officer with ties to the KGB who "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."

President Reagan wasn't sure, and in March, 1984, asked Arthur Hartman, his ambassador to the Soviet Union, "Do you think Soviet leaders really fear us, or is all the huffing and puffing just part of their propaganda?" We don't know what Hartman said in response, but John McMahon, the CIA director at the time, believed the Soviets were simply "rattling their pots and pans" to stop further Pershing II deployments.

It's unclear how much of the fear was just pots and pans. Jones writes that although "real-time analysts, retroactive re-inspectors, and the historical community may be at odds as to how dangerous the War Scare was, all agree that the dearth of available evidence has made conclusions harder to deduce." Jones did not get all the information he asked for. (The complete list of unclassified documents are collected at the Archives' website, with two more sets of documents to follow.) The NSA told him it had 81 more documents, but did not release them. However, it did "review, approve for release, stamp, and send a printout of a Wikipedia article," he noted.

Still, we do have more evidence of serious Soviet preparations. Documents obtained by Jones detail a massive KGB intelligence-gathering mission called Operation RYaN. (The name is a Russian acronym for "nuclear missile attack.") According to the CIA article, RYaN was "for real" and accelerated in the early 1980s during the scare. The goal was to find out if and when the United States and NATO would attack. According to KGB instructions sent to agents in London, Soviet spies were to monitor bomb shelters, blood banks, military bases and key financial and religious leaders for signs of war preparations. "Many of the assigned observations would have been very poor indicators of a nuclear attack," Jones warns.

But in another sense, the scrambling for any scrap of intelligence – whether good or bad – reflected a feverish belief among some quarters that war was just around the corner. "[T]he Reagan administration marked the height of the Cold War," notes one declassified history published by the National Security Agency. "The president referred to the Soviet Union as the Evil Empire, and was determined to spend it into the ground. The Politburo reciprocated, and the rhetoric on both sides, especially during the first Reagan administration, drove the hysteria. Some called it the Second Cold War. The period 1982-1984 marked the most dangerous Soviet-American confrontation since the Cuban Missile Crisis."

Worse, there were "a lot of crazy people" in the Kremlin and Soviet military command, according to Vitalii Tsygichko, an analyst for the Soviet General Staff who was interviewed by the Pentagon. "I know many military people who look like normal people, but it was difficult to explain to them that waging nuclear war was not feasible. We had a lot of arguments in this respect. Unfortunately, as far as I know, there are a lot of stupid people both in NATO and our country."

Considering the consequences of a war, and how close it came, those comments certainly ring true.