One of the Cold War’s great mysteries is how the world survived the second week of November 1983.

That it did is in large part thanks to the actions—or, more accurately, the inaction—of an Air Force officer named Leonard Perroots, who died this January. That it almost did not was a function of Ronald Reagan’s rhetorical and military bellicosity, the Soviets’ fear of that aggressiveness, and a tragicomic degree of misperception. At no other point in history had two nations devoted the level of human, financial, and technical resources that the United States and the Soviet Union did to sussing each other’s intentions. And yet their confusion remained so total that the Soviets mistook a NATO war game for the prelude to an actual attack, even as Reagan thought he was doing his utmost to pursue peace.

For decades, the U.S. government kept whole chapters of this near-catastrophe secret, but the lessons of that fraught autumn are finally coming into focus. And not a moment too soon.

Few would mistake Donald Trump for Ronald Reagan, yet the parallels to the early 1980s are striking. With contradictory statements about both expanding and reducing U.S. nuclear forces, Trump has sown strategic uncertainty among friends and foes alike at a moment when the United States and Russia are modernizing their nuclear arsenals. On the heels of Russia’s annexation of Crimea, the Baltic nations have stepped up preparations for war and NATO has moved military forces to vulnerable areas in the East. The Kremlin has called U.S. missile defenses “a certain threat to the Russian Federation,” and Europe is once again in range of intermediate-range nuclear missiles after Moscow deployed a new weapons system that violates a pivotal 30-year-old treaty. Meanwhile, Russia has repeatedly sent warplanes to probe European airspace and harass U.S. Navy ships, and operations in Syria have put the U.S. and Russian militaries in dangerous proximity to one another. The recent U.S. missile strike against Syria’s Russian-backed air force has only heightened the risk of a miscalculation. As Mikhail Gorbachev recently wrote, “It all looks as if the world is preparing for war … the nuclear threat once again seems real.”

Still, neither side wants war, so why are so many experts concerned that the United States and Russia could stumble into one? To understand how combustible the current moment is, it helps to revisit what happened in 1983.

Perhaps the most perplexing strategic question of the 20th century was how to prevent a nuclear war—and what to do if those efforts failed.

By the early 1970s, the Soviets had realized that a nuclear war would not be winnable in any meaningful sense. Some American weapons would survive any attack, and even one modern thermonuclear warhead could obliterate Moscow. At the same time, the Soviets saw an advantage, however Pyrrhic, to striking first—the more enemy weapons that one could destroy, the fewer that could be used against them. A U.S. National Intelligence Estimate written in 1987 and recently made public for the first time, put it this way:

[The Soviets’] nuclear warfighting strategy … does not predispose them to exercise restraint if they saw inherently high risks that global nuclear war could occur and believed restraint on their part could jeopardize their chances for effectively waging such a war. The Soviets have strong incentives to preempt in order to maximize the damage to US forces and limit damage to Soviet forces and society.

In other words, not only did the Soviets keep their forces ready to respond immediately to a U.S. launch (a dangerously reflexive policy that the United States and Russia maintain even today), they planned to fire their missiles if they thought the United States was preparing a strike—sort of an atomic sucker punch.

“Winning” was a fiction, but it was not clear that Reagan understood that.

By the time Reagan became president, the Soviet Union feared it might soon be necessary to throw that punch. Jimmy Carter had taken a number of provocative steps—approving the mobile land-based MX nuclear missile, increasing defense spending, and calling for improvements to the U.S. ability to fight and, by implication, win a nuclear war. Like the Soviets, American strategists understood that “winning” was a fiction, but if deterrence failed, they wanted options. Unfortunately, such options looked sinister to the Soviets.

The so-called Euromissiles were the final straw. In the late 1970s, the Soviets began fielding intermediate-range SS-20 nuclear missiles in the western Soviet Union—missiles that could potentially destroy NATO’s European forces in a pre-emptive strike. In response, Carter backed a NATO decision to forward-deploy nuclear-armed Gryphon cruise missiles and Pershing II ballistic missiles in several European countries. The Pershing IIs could reach Russia in less than 10 minutes, and the Gryphons could reach Moscow in under an hour, likely undetected by Soviet radar. Herb Meyer, the vice chairman of the National Intelligence Council, noted that the time it would take these missiles to hit their targets was “roughly how long it takes some of the Kremlin’s leaders to get out of their chairs, let alone to their shelters.”

Photo illustration by Slate. Photos by National Security Archive and Soviet Military Power magazine via National Security Archive.

To the United States, the missiles were a sign of solidarity with European allies and an attempt to get the Soviets to pull back the SS-20s. But the Soviets saw them as first-strike weapons, designed to decapitate the Soviet leadership before it could get off a shot. Mikhail Gorbachev would later describe the missiles as “a gun pressed to our temple.”

The KGB tasked an internal think tank with detecting Western preparations for a first strike. The result was a massive spy effort dubbed Operation RYaN—“RYaN” being the Russian acronym for “nuclear missile attack.” RYaN tasked some 300 operatives with examining 292 different indicators—everything from the location of nuclear warheads to efforts to move American “founding documents” from display at the National Archives. According to declassified Stasi documents, the resulting data was then fed into a primitive computer system, which attempted to calculate whether the Soviets should go to war to pre-empt a Western first strike.

Soviet fears shot up after Reagan took office. Reagan had no desire to fight a nuclear war—in fact, he hoped to engage the Soviets with what he described in his diary as a “quiet diplomacy approach”—but in the early years of his presidency, this hope for dialogue repeatedly clashed with his denunciations of the Soviet system. For example, in February 1983, at the suggestion of his pragmatic secretary of state, George Shultz, Reagan met for two hours with Soviet Ambassador Anatoly Dobrynin. But just weeks later, he launched his most famous rhetorical assault against the Soviet Union, labeling it an “Evil Empire” in a speech to the National Association of Evangelicals.

Reagan did not see these approaches as contradictory. He believed that candor about America’s intentions, which he saw as self-apparently noble, and candor about Soviet intentions, which he believed were obviously nefarious, would show the communists that aggression was futile and that their system of government was ultimately doomed, thus ending the Cold War. It is one of the Reagan administration’s greatest ironies that his effort at clarity so confused the Soviet Union that in 1983 its leaders became convinced the United States was planning a nuclear first strike.

It’s not hard to see why. Since Reagan had assumed the presidency, he had vowed to “rearm” America by doubling the defense budget within five years. He authorized development of the B-1 bomber and the neutron bomb. He ordered the deployment of an additional 3,000 nuclear weapons. And he sped up programs to build the Trident II submarine-launched ballistic missile, a nuclear-capable sea-launched cruise missile, and the B-2 stealth bomber.

Even more provocative were his reported plans for using those weapons. A Pentagon planning document leaked to the New York Times in May 1982 said the United States must be able to wage nuclear war over six months, with pauses for reloading missile silos and firing new salvos, and still have enough nukes to deter or fight a second nuclear war. In August 1982, the Los Angeles Times reported that Reagan had signed a presidential order calling for the ability to win a nuclear war.

Again, “winning” was a fiction, but it was not clear that Reagan understood that. In 1982, he asked Congress for $4.3 billion for a civil defense effort that would supposedly protect hundreds of millions of Americans from a nuclear attack. Then, on March 23, 1983, in his nationally televised “Star Wars” speech, Reagan called for a defensive shield that would render nuclear weapons “impotent and obsolete.” Reagan said he wanted only “to save lives [rather] than to avenge them,” but a missile shield, if it worked, would have left the Soviets vulnerable to U.S. attack.

Some Soviet war planners concluded that the United States might be plotting war.

By this time, Yuri Andropov, formerly head of the KGB, was general secretary of the Soviet Union. Andropov is often painted as paranoid, but a recently declassified CIA psychological profile described him as a “sophisticated man … probably better informed on foreign affairs, and on at least some domestic matters than any other Soviet party chief since Lenin.” Now, this sophisticated man believed that he might have to start a nuclear war in order to save his country. He tried to convey his anxieties to the Americans, raising the threat of nuclear war four times during a meeting in June 1983 with Averell Harriman, who had served as FDR’s ambassador to the Soviet Union and whom Reagan had dispatched to Moscow to “size up” Soviet intentions.

Instead of trying to reassure the Soviets, the Reagan administration had been using the American military to keep them off balance. U.S. warships had been sneaking close to Soviet shores and then launching aircraft that would head toward Soviet airspace, only turning back at the last minute. The Soviets would be forced to scramble their own jets in response. According to a once-classified Cold War history written by the National Security Agency, “[T]hese actions were calculated to induce paranoia, and they did.”

Photo illustration by Slate. Photos by National Security Archive and Joe Dobbels/DOD.

These operations culminated in April and May of 1983, when three U.S. carrier battle groups comprising 40 ships undertook a massive exercise in the Pacific designed to simulate all-out war against Soviet forces. Navy aircraft even conducted a mock bombing run over a Soviet military site on the Pacific island of Zeleny. The Soviets issued a demarche over the incident, and Andropov himself responded by issuing a “shoot to kill” order if U.S. forces ever crossed into Soviet territory again.

That order had deadly consequences. In the early morning hours of Sept. 1, Soviet radar picked up what it thought was an American spy plane crossing into Soviet airspace. “Given the paranoia that had existed since April,” the NSA history reports, “it was unthinkable that such a penetration could be permitted without action.” So an Su-15 fighter was dispatched to shoot it down.

Unfortunately, the “spy plane” was a 747 passenger jet—Korean Air Lines Flight 007—bound for Seoul. All 269 people on board died. Among them were 62 Americans, including a sitting U.S. congressman, and 22 children younger than 12.

Reagan called the incident “an act of barbarism.” But his accusation that the Soviets had intentionally murdered hundreds of civilians ran counter to intelligence reports. The National Security Agency concluded—in a recently released document—that the Soviets had genuinely believed KAL 007 was a spy plane: “It was the Reagan people who insisted that the Soviets could not have mistaken a 747 for a [spy plane].” Perhaps, the alternative explanation was just more frightening. As Reagan wrote in his memoirs, “If, as some people speculated, the Soviet pilots simply mistook the airliner for a military plane, what kind of imagination did it take to think of a Soviet military man with his finger close to a nuclear push button making an even more tragic mistake?”

Photo illustration by Slate. Photos by TASS via Getty Images and DOD/National Security Archive.

It was at this delicate moment that the United States and its allies decided to simulate a nuclear attack on the Soviet Union and its Warsaw Pact allies. The simulation, dubbed Able Archer 83, was part of a war game orchestrated by NATO headquarters in Brussels, the British Ministry of Defense, and the Pentagon. Practiced by allied soldiers across Europe, its objective was to “practice command and staff procedures, with particular emphasis on the transition from conventional to non-conventional operations, including the use of nuclear weapons.”

The simulation included a backstory, a narrative of how the West believed nuclear war could begin. It started with a sudden change of leadership in the Kremlin. Simmering proxy wars in Syria, Iran, and South Yemen devolved into actual East-West conflict after nonaligned Yugoslavia shifted its allegiance to the West. According to the scenario, Soviet forces invaded Finland on Nov. 3 and Norway on Nov. 4. The ground war soon engulfed Europe. By Nov. 4, Warsaw Pact forces had invaded West Germany.

Then, on Nov. 7, Able Archer 83 began with NATO officers—stationed in Belgium, Washington, London, in the European field, and at allied air bases—rehearsing how they would fight a nuclear war. They practiced trans-Atlantic and encrypted communications procedures, ran through nuclear loading and handling procedures, and even donned protective equipment and moved to an “alternative war headquarters” to simulate operating under biological, chemical, and nuclear attack.

What if the Soviets actually think we’re going to launch nuclear weapons and we’re disguising it as an exercise? What if they launch against us?

According to the exercise’s scenario, on the morning of Nov. 8, unable to repel the enemy advance with conventional forces, NATO war gamers requested permission for “initial limited use of nuclear weapons against pre-selected fixed targets.” They first attempted to signal NATO’s resolve by destroying a Soviet city. (Both sides embraced the idea of “nuclear signaling” during the Cold War. According to David Abshire, the U.S. ambassador to NATO at the time, the Americans’ preferred “signal city” was Kiev and the Soviets’ was Boston.)

Soldiers in the field practiced preparing their weapons, and Western capitals granted NATO permission to “destroy” Eastern European cities with nuclear attacks. But even this did not stop the enemy. As a result, on Nov. 10, NATO’s military chief requested a “follow-on use of nuclear weapons.” Washington—and the other capitals—approved this request, and on Nov. 11, the order was given and the attack was practiced in the field. A full-scale nuclear war had broken out, and Able Archer 83 ended.

Able Archer 83 is sometimes described as merely a command-post exercise—that is, an obvious drill that could not have been interpreted as anything else—but it did not feel that way to the U.S. Air Force personnel involved. One technical sergeant, who drew from an actual target database during the game, described the exercise as “starkly realistic.”

That assessment is backed up by numerous declassified documents, which report a series of “special wrinkles” that set Able Archer 83 apart from other war games. One report notes that, before the exercise began, NATO put its forces on simulated general alert—the level of readiness one would use during an actual war. Another report noted that “some US aircraft practiced the nuclear warhead handling procedures, including taxiing out of hangars carrying realistic-looking dummy warheads.” (The technical sergeant claims that real warheads were used.) Other documents show that in the run-up to Able Archer 83, the United States deployed 16,000 additional troops to Europe on radio-silent flights. Perhaps most significantly, NATO officers practiced new procedures for ordering the use of nuclear weapons.

Air Force personnel worried that all this realism might give the impression that NATO was preparing a real attack. According to Tod Jennings, who, as a staff sergeant, spent Able Archer 83 in a bunker outside Oslo, relaying nuclear orders via teletype, the exercise was realistic enough that he and his fellow airmen began asking, “What if the Soviets actually think we’re going to launch nuclear weapons and we’re disguising it as an exercise? What if they launch against us?”

According to the Air Force technical sergeant, during previous nuclear-release exercises, war-gamers would purposefully end on an anti-climax: They would “do a fade out, a de-escalation, winding it down and making it disappear.” By contrast, during Able Archer 83, “I think there was something from the administration that said let’s just ratchet this up a little bit more. Guess what? They got the response 10 times more than what they expected.”

Photo illustration by Slate. Photo by Aerospace AV Service/DOD/National Security Archive and DOD/National Security Archive.

According to a 1990 review of the exercise by the President’s Foreign Intelligence Advisory Board that was only declassified in 2015 after a 12-year battle by the National Security Archive, “the Soviets implemented military and intelligence activities that previously were seen only during actual crises.”* These activities included an “unprecedented” 36 intelligence flights, “probably to determine whether US naval forces were deploying forward in support of Able Archer 83.” The Soviet military response was also “unparalleled in scale” and included “transporting nuclear weapons from storage sites to delivery units by helicopter.” All flight operations except intelligence collection flights were suspended “probably to have available as many aircraft as possible for combat.” The PFIAB report concluded that this response “strongly suggests to us that Soviet military leaders may have been seriously concerned that the US would use Able Archer 83 as a cover of launching a real attack.”

Multiple high-level Soviet officers have confirmed in interviews that Soviet nuclear missile forces were placed on “raised combat alert” during Able Archer 83. At least one account claimed that the alert reached the highest levels of the Soviet military and that Marshal Nikolai Ogarkov, chief of the general staff, monitored events from a bunker outside of Moscow. And the U.S. technical sergeant says he received daily briefings showing that the Soviets were increasing their level of military readiness and upping the operational tempo of their forces: “Their posturing [was] rising in some cases by the hour if not the minute.” The United States and the Soviet Union had started climbing the escalatory ladder toward war. The question is why they stopped.

This is where the role played by Leonard Perroots, a military intelligence officer, was so critical. At the time of Able Archer 83, the West Virginia native with almost 30 years of service was assistant chief of staff for intelligence for the U.S. Air Force in Europe. While overseeing Able Archer 83, he noted that Soviet forces (the real ones, not the simulated ones) were raising their alert levels. But, instead of responding in kind, Perroots did nothing. Had he elevated the alert level of Western military assets—which would not have been an unreasonable thing to do—the Soviets might well have concluded that the exercise was indeed cover for an attack. Instead, Perroots, acting on instinct, saw that doing nothing would halt any climb up the escalatory ladder. It ended what would become known as the “war scare” and possibly averted a nuclear exchange.

His calm response aside, Perroots was shaken by the episode. So, in January 1989, just before retiring as head of the Defense Intelligence Agency, he wrote a letter recalling the danger he experienced during Able Archer 83 and outlining his disquiet that the U.S. intelligence community did not give adequate credence to the possibility that the United States and Soviet Union came unacceptably close to nuclear war during Able Archer 83. He sent this letter to the President’s Foreign Intelligence Advisory Board, which was shocked into action. Its comprehensive report was based on hundreds of documents and more than 75 interviews with American and British officials.

The PFIAB authors wrote that they hoped their report would prompt “renewed interest, vigorous dialogue, and rigorous analyses of the [war scare].” But the report’s high level of classification, and the U.S. government’s sluggish declassification process, meant that we have just started to appreciate its conclusions about the dangers of nuclear war through miscalculation. When Perroots died this January, his letter to the PFIAB was still classified—28 years after he had written it.

No one knows precisely how close the world came to nuclear war in 1983, but Reagan soon changed his approach to the Soviets. On Nov. 18, he wrote in his journal, “I feel the Soviets are so defense minded, so paranoid about being attacked that without being in any way soft on them we ought to tell them no one here has any intention of doing anything like that.”

When Gorbachev took power in 1985, the American president worked with the Soviet reformer to eliminate intermediate-range nuclear weapons, including the Pershing IIs, Gryphons, and SS-20s. More arms reductions followed. Today, the New START agreement caps each country’s nuclear arsenal at 1,550 deployed strategic warheads and 700 deployed missiles and bombers. Able Archer 83 marked a turning point for relations between Washington and Moscow.

Unfortunately, they have turned back. Russia’s annexation of Crimea and its invasion of Eastern Ukraine have evoked memories of Soviet-era imperialism, and its military buildup in the West, numbering some 330,000 troops, combined with its deployment of nuclear-capable Iskander missiles to Kaliningrad, threatens the Baltics. In response, NATO announced in October that it would deploy thousands of troops to the East to deter aggression. Then, in February, the New York Times reported that Russia had deployed a new cruise missile that violates Reagan and Gorbachev’s hard-earned ban on intermediate-range forces—a development later confirmed by the vice chairman of the Joint Chiefs of Staff.

For his part, Trump—whose temperamental volatility made his fitness to command U.S. nuclear forces an issue during the campaign—has cultivated strategic uncertainty with conflicting, confusing, and sometimes bellicose statements. In December, he tweeted that the United States must “greatly strengthen and expand its nuclear capability” and, when asked whether that might not trigger an arms race, blithely responded, “Let it be an arms race. We will outmatch them at every pass and outlast them all.” The next month, he called for nuclear reductions with Russia, but in February he told Reuters that U.S. nuclear forces needed to be “at the top of the pack” and derided New START as a “bad deal.”

Nuclear miscalculation and escalation are also possible with adversaries besides Russia—most notably North Korea, which has worked for decades to build an arsenal of nuclear missiles. Currently, the United States and South Korea are conducting a massive war game involving tens of thousands of troops. Much like the Soviets in the 1980s, the North Koreans worry such games are just a rehearsal for an attack. In response, they have threatened pre-emptive nuclear strikes and, as nonproliferation expert Jeffrey Lewis has written, are practicing for war by conducting missile-launch exercises. Making matters significantly worse, both U.S. and North Korean doctrine call for the first use of nuclear weapons in a conflict, meaning that the chance of a spark turning into a nuclear conflagration is extremely high.

Able Archer 83 is a reminder not only that nuclear weapons are one of the few existential threats the United States and its allies face, but also that humility is key to the conduct of national security policy. The United States assumes that it is clear in communicating its intentions and understanding those of its enemies. It also tends to assume that it controls the consequences of its actions. Neither of these things is necessarily true. Misperception, chance, and accident are facts of history. In 1983, war was averted because of restraint. Unfortunately, President Trump is not known for self-control.

*Correction, May 4, 2017: This post originally misstated that the review of the exercise by the President’s Foreign Intelligence Advisory Board was released in 1991. It was 1990.

Published April 13, 2017, 5:45 PM ET