Remember Reykjavik. That’s my advice to those panicking over President Trump’s decision to walk away from President Obama’s nuclear deal with Iran.

Reykjavik was the site of a summit in Iceland where, in October 1986, a nuclear-arms deal was proffered by the Soviet party boss, Mikhail Gorbachev. Ronald Reagan stunned the world by walking out.

It turned out that his move set up our victory in the Cold War.

Reagan faced enormous pressure from a nervous world to get a deal from the Soviet camarilla. Just the way the Europeans have been pressuring Mr. Trump to stick with the Iran appeasement.

France’s president, Emmanuel Macron, went to Congress to plug for appeasement. Britain’s foreign secretary, Boris Johnson, made a plea to Trump, as did Germany’s chancellor, Angela Merkel.

They were prepared to improve — or expand — the deal. They were, though, desperate for Mr. Trump to stay in the confounded compact, lest we irk the Iranians.

So when Mr. Trump walked, advocates of appeasement went into a full-bore panic. Mr. Obama himself said Mr. Trump was “misguided.” The Washington Post said he “brought us closer to war.”

The Europeans expressed “regret and concern.” The ink was barely dry on Trump’s memorandum when The New York Times started carrying on about how there’s no “Plan B.”

That’s just what the Times did when Reagan — in a move that caught even his own staff flat-footed — stood up and walked out of Reykjavik. “What happens next?” the Times asked.

Not even Reagan knew, of course. He did know that he wasn’t prepared to do a deal that wasn’t right for America and was contrary to his campaign promises.

Those points are captured in an evocative short film about Reykjavik that’s part of PBS’s “American Experience.” It shows the incredible intimacy of that summit.

It took place in an isolated building, nothing like the palaces favored by our ex-secretary of state, John Kerry. Reagan, Gorbachev, two translators and a few aides were crammed into a tiny room.

The issues, though, were huge. The Soviets, as PBS relates, suggested both sides destroy half of their long-range bombers and missiles and eliminate all missiles threatening Europe.

It would have been a major de-escalation in the Cold War standoff. Plus, Gorbachev offered to make human rights a regular part of the agenda. Secretary of State George Shultz called it “a breakthrough.”

The next morning, though, Gorbachev had one catch. He wanted us to restrict our work on our Strategic Defense Initiative anti-missile defense system: No testing.

Reagan refused. His chief of staff, Don Regan, suddenly sensed the president was growing restless, wanting to get back to Washington in time for dinner with Nancy.

Restricting SDI to the lab seemed an odd condition. Particularly after America countered with a proposal to ban space defenses for a decade, during which both sides would eliminate all their long-range nuclear missiles.

The leaders plunged way past their handlers and experts. Yet Gorbachev clung to the condition of restricting what earned the nickname “Star Wars.”

That would doom SDI, Richard Perle whispered to Reagan. The president leaned over to his chief of staff. If we agree, he said, “won’t we be doing that simply so we can leave here with an agreement?”

“Within seconds,” Mr. Perle recalled, “it was over.”

When Reagan slumped into his limousine, he was dejected — as if, Don Regan recalled, he’d “just lost a combination of the Rose Bowl, the Stanley Cup, and the olympics.”

The president, though, was adamant about not giving up Star Wars. “I couldn’t do that,” he told Regan, “I promised the American people I would not give in on that. And I cannot do it.”

And he was right — the Soviets’ so feared the arms race that they spent themselves broke trying to compete.

Which brings it back to Mr. Trump. Like Reagan, he made a promise on Iran to the American people and clearly felt bound. Redeeming that promise would be itself a boost to our credibility.

It’s not just the Iranians who are watching, after all. Keeping his word is the best thing Mr. Trump could do as he pivots to Kim Jong Un. Or to Communist China or Vladimir Putin’s Russia.

There’s no guarantee that Iran, North Korea or any adversary will fold its tent. There was no guarantee that the Soviet Union would, either. After Reykjavik, it took them three years.

This column first appeared in the New York Post.