A NATO war game had the Soviet Union readying for nuclear war in a terrifying Cold War close call, new documents show

• New documents have offered insights into the 1983 incident that could have started a nuclear war.
• A routine NATO military exercise triggered a "war scare" and the Soviets were readying their forces for battle.
• The Soviets were making preparations for the "immediate use of nuclear weapons," the documents revealed.

During the Cold War, the US and the Soviet Union came dangerously close to nuclear-armed conflict on numerous occasions. New documents offer insight into just how close the two countries came to war as NATO drilled in Europe in November 1983 - it is said to be the closest the two sides came to conflict since the 1962 Cuban Missile Crisis.

The routine NATO nuclear command and control exercise "Able Archer 83" alarmed the Soviets, but the US wasn't aware to what degree until much later.

For months after the military exercise, the intelligence community did not believe a "war scare" happened, and it was the prevailing assessment that the Soviets were not worried about a nuclear attack and that any unusual Soviet activity during the 1983 drill was unrelated.

Later evaluations of relevant intelligence, since declassified, show that was not the case. The documents also show that the US underestimated Soviet concerns and the risk of a nuclear response, leaving it unaware that seemingly benign actions could have had dire consequences.

Recalling the intelligence coming in during the Able Archer exercise, Lt. Gen. Leonard H. Perroots, the assistant chief of staff for intelligence at US Air Forces Europe, wrote in a recently-declassified 1989 memo that Soviet fighter-bomber divisions stationed in East Germany were put on 24-hour alert, and one squadron in each regiment was armed to "destroy first-line enemy targets" and placed on a 30-minute alert.

Another intelligence report said that at least one Soviet squadron in the area had requested and received permission to reconfigure their aircraft in a atypical manner, leading to analyst speculation that "this particular squadron was loading a munitions configuration that they had never actually loaded before, i.e., a warload."

These developments struck Perroots, a US intelligence officer on site for the exercise, as unusual, so he contacted Gen. Billy Minter, commander in chief of US Air Forces Europe. The general asked him if he thought the US military should increase real force generation in preparation for conflict.

Perroots said that "we should carefully watch the situation," explaining that there was "insufficient evidence to justify increasing our real alert posture."

A 1990 President's Foreign Intelligence Advisory Board investigation report triggered by Perroots' memo and declassified in 2015 said that "following the detection of Soviet Air Forces' increased alert status, it was his [Perroots's] recommendation, made in ignorance, not to raise US readiness in response-a fortuitous, if ill-informed, decision."

Perroots' recommendation was based on an incomplete intelligence picture, but it appears to have helped the US and Soviet Union avert nuclear war.

"The military officers in charge of the Able Archer exercise minimized the risk by doing nothing in the face of evidence that parts of the Soviet armed forces were moving to an unusual alert level," the PFIAB report said.

"But," the report reads, "these officials acted correctly out of instinct, not informed guidance."

"If I had known then what I later found out I am uncertain what advice I would have given," Perroots wrote in his memo.

The general asked: "What might have happened that day in November 1983 if we had begun a precautionary generation of forces rather than waiting for further information?"

After the Able Archer exercise had already ended, the general learned that all units of the Soviet 4th Air Army had been put on high alert in reaction to the NATO drill, and the heightened alert state "included preparations for the immediate use of nuclear weapons."

Additionally, the Soviet military secretly ordered a "standdown" for aviation units, halting routine flying activity as air assets were repurposed. Western intelligence did not detect the change until almost a week later, when fully-armed Flogger fighters were spotted on alert at a base in East Germany.

The "ominous" Soviet posturing became "more frightening" as additional intelligence came in a couple of years later revealing that at the time of Able Archer 83, Soviet intelligence analysts were convinced that the West was planning a surprise nuclear attack and that an exercise like Able Archer would provide the perfect cover.

The 1990 PFIAB investigative report that followed Perroots' memorandum stated that the US may "have inadvertently placed our relations with the Soviet Union on a hair trigger."

But, due to the challenges of gathering intelligence, this was not clear during the Able Archer exercise.

"For Western collectors the context was peacetime without even the most basic ripples of crisis," Perroots wrote in his memo. "For the Soviets, however, the view may have looked quite different."

"The US intelligence community did not at the time, and for several years afterwards, attach sufficient weight to the possibility that the war scare was real," the PFIAB report said. "As a result, the President was given assessments of Soviet attitudes and actions that understated the risks to the United States."

"Moreover," the report explained, "these assessments did not lead us to reevaluate our own military and intelligence actions that might be perceived by the Soviets as signaling war preparations."

The Able Archer 83 incident is one of several Cold War close calls. It followed another on Sept. 26, 1983 in which a Soviet nuclear early warning system indicated that the US had launched nuclear-armed intercontinental ballistic missiles. Rather than treat the apparent launch as a real threat, Soviet officer Stanislav Petrov made the decision to wait for more information.

The warning turned out to be a false alarm, but the wrong decision in that moment could have been disastrous.

Hans Kristensen, a nuclear weapons expert at the Federation of American Scientists, told Insider that these incidents should serve as an important reminder as great power competition heats up between the US, Russia, and China.

"Once countries embark on these aggressive power plays with great power competition like we're in the middle of now, we have to be extraordinarily careful because we cannot in the nature of things predict where things will end up," he said.

While he acknowledged that current tensions do not compare to those of the Cold War, he said it is worth it to pay attention to the parallels.