As President Trump prepares to announce Tuesday his decision in respect of the Iran appeasement, our advice is this: Remember Reykjavik. That was the greatest moment of Ronald Reagan’s presidency. It was the summit at which Reagan faced down the Soviet party boss Mikhail Gorbachev, rejected Russ wheedling against our Stars Wars missile defense, stood up, and walked out.

That’s just what President Trump ought to do to the Mullahs, who are pulling every string they can — and making every threat they can think of — to keep Mr. Trump from walking away from the Iran deal. As Mr. Trump was working on his decision, President Obama’s erstwhile state secretary, John Kerry, was meeting directly with the Iranian foreign minister to try to keep Mr. Trump in the deal.

The best that could be said for Mr. Kerry’s collusion with the Iranians is that it’s par for the Kerry course. Meantime the French president, Emmanuel Macron, has been lobbying for a scheme to fix and expand the deal. On Monday, no less a figure than Britain’s foreign minister, Boris Johnson, fetched up on the op-ed page of Monday’s New York Times with a long plea, also siding with Iran. Et tu, Boris?

It may be that all this frantic pleading is too late, in that Mr. Trump himself has suggested he’s already made up his mind. The Times on Monday evening was reporting that “diplomats who were familiar with the negotiations” were claiming Mr. Trump was, as the Times paraphrased them, “inclined to scrap the deal” and re-impose sanctions that the articles of appeasement had suspended.

Then again, too, the Times suggested that it is “unclear” whether the president would “moderate that move” by “allowing the European nations to move ahead with their economic relations with Tehran without being penalized by the United States.” Our own view is that the harder line Mr. Trump takes with the Iranians, and their apologists at Europe, the better. He can take a lesson from the Gipper.

All kinds of issues were on the table at Iceland in October of 1986 — human rights, Soviet Jews, missile reduction. The Soviet regime was forthcoming on some of it. Reagan, though, wanted Star Wars, the missile defense Reagan thought that we could construct and understood that the Soviets couldn’t. Mr. Gorbachev, of course, understood just what Reagan grasped, and refused. Hence Reagan’s walkout.

The thing to remember about the moment is the weeping and wailing that it precipitated on the left. It was right up there with Messrs. Macron, Johnson, and Kerry. Some of us worried at the time that the European leaders would have a nervous breakdown. And yet, what? Reykjavik was in October 1986. Three years later, the Berlin Wall came down and the Soviet collapse had begun

Could that happen in Iran? Could the regime eventually collapse if Mr. Trump stands his ground? The clearest arguments have come from Prime Minister Netanyahu, who stresses not only Iran’s nuclear work and past lies but also the importance of sanctions to keep from Iran the vast billions it needs for its expansionist war in the Middle East. Understanding such dynamics is how Reagan won the Cold War.