If there is a secret file drawer at the White House that contains lessons left by previous presidents, President Trump should thumb through them for a folder labeled “Reykjavik.” The history of that remarkable weekend in Iceland, Oct. 11-12, 1986, when President Ronald Reagan and Soviet leader Mikhail Gorbachev improvised their way — almost — to eliminating their nuclear arsenals, is worth examining now as the president heads toward meeting North Korean leader Kim Jong Un.

Reagan went to Iceland as a self-confident negotiator, eager to use his formidable powers of persuasion. He had overprepared for a summit the year before in Geneva, but this time around, he hardly prepared at all. There was no precooked agenda, as in previous superpower summits. A Soviet specialist at the State Department wrote a memo that opened: “We go into Reykjavik next week with very little knowledge of how Gorbachev intends to use the meeting.” The president’s national security adviser, John M. Poindexter, told Reagan to “anticipate no substantive agreements per se,” adding that the “meeting is in no sense a substitute or surrogate for a summit.” Reagan wrote in his diary, “This would be preparatory to a Summit.”

Gorbachev had proposed a “quick one-on-one meeting” that wouldn’t necessarily last more than a day. In fact, the wily Gorbachev had other ideas. He had a plan to first offer radical reductions in offensive nuclear weapons, and then demand that Reagan slow down his ballistic missile-defense research effort, the Strategic Defense Initiative, commonly referred to as Star Wars. “The main goal of Reykjavik,” wrote Gorbachev’s adviser Anatoly Chernyaev before the meeting, “is to sweep Reagan off his feet . . . by our bold, even ‘risky’ approach.”

Gorbachev did just that. The summit went into overtime: all-night staff negotiations, then a second day of talks. Gorbachev offered the startled Reagan concession after concession, including the dismantling of hundreds of nuclear weapons. “We were getting amazing agreements,” Reagan later recalled in his memoir. “As the day wore on, I felt something momentous was occurring.” It was. They had scratched out on paper a 10-year period of nuclear disarmament. “Let me ask this,” Reagan inquired at one point of Gorbachev. “Do we have in mind — and I think it would be very good — that by the end of the two five-year periods all nuclear explosive devices would be eliminated, including bombs, battlefield systems, cruise missiles, submarine weapons, intermediate-range systems, and so on?” Gorbachev replied, sure, we can make a list. Reagan, who had long dreamed privately of nuclear abolition, was astounded. Secretary of State George P. Shultz urged him on: “Then let’s do it.”

Then Gorbachev sprung the trap, demanding a limit on Star Wars . Reagan refused, and the summit fell apart.

Reagan had sharp instincts, but it was risky and rather foolhardy to dismantle the nuclear arsenal without preparation. In the days that followed Reykjavik, there was a public disagreement with Moscow about exactly what had been worked out. It turned out the Soviet version was correct. Then, it was revealed Reagan had failed to consult the chairman of the Joint Chiefs of Staff, Adm. William J. Crowe Jr., who protested that the cuts would pose “high risks to the security of the nation.” Reagan was nonplussed, and always focused on the big picture. He headed out on the campaign trail to boast that when Gorbachev tried to stop the Strategic Defense Initiative, “I just said, No!” Crowds loved it.

The lessons? Know your adversary’s intentions and capabilities before stepping into the room. That is why we have intelligence agencies, diplomats and scholars. Listen to them. If you are going to sit face-to-face with your adversary, don’t just wing it. Improvisation can be smart and effective; but behind any winning performance, there must be serious staffing to make sure gains are for real, and can be sustained — especially with an adversary known for breaking deals.

At the news conference Gorbachev gave after Reykjavik, many journalists, myself included, believed the summit was a failure. But looking back, the wild and unscripted nature of the meeting took Reagan and Gorbachev far. They probably never would have had that discussion otherwise. Had Reagan been better prepared, could he have closed a deal at Reykjavik? We will never know. Fortunately, a good portion of the arms reductions they envisioned did come about later, with real staff work, negotiations and treaties, and because the Cold War ended.

The lesson for Trump is not to keep instincts in check so much as to keep danger in check. That means thinking ahead, careful preparation, and not going it alone.

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