Despite muhc heated rhetoric, the Trump administration is doing little to counter Iranian aggression. In Syria, its strategy of striking deals with Russia has opened the way for Tehran’s forces to establish control over a corridor between Damascus and Baghdad. In Afghanistan, Iran is steadily building a strategic position even as President Trump balks at a plan to strengthen U.S. support for the Afghan government. In Yemen, the United States enables its Persian Gulf allies to pursue an unwinnable proxy war with Tehran whose main result has been the world’s worst humanitarian crisis.

In only one area has the Islamic Republic’s toxic ambition been relatively contained: the production of material for use in nuclear warheads. According to international inspectors and the U.S. intelligence community, Iran has largely abided by the terms of the 2015 nuclear deal, which greatly reduced its stockpile of enriched unranium and placed strict limits on its nuclear activities. If the regime continues complying, it could be a decade or more before Iran could again threaten to become a nuclear power. Yet perversely, Mr. Trump is matching his passivity toward Iran’s regional meddling with an apparent determination to torpedo the nuclear pact.

After grudgingly certifying in July that Iran was meeting the terms of the deal — a test mandated by Congress every 90 days — Mr. Trump told the Wall Street Journal that he planned to find the regime noncompliant when the next certification is due in October. How to reach such a finding if the intelligence community judges otherwise? According to Foreign Policy, Mr. Trump ordered a group of political aides, including now- fired strategist Stephen K. Bannon, to cook up a rationale — something that presumably will be made easier by their lack of data or expertise.

The real experts puzzle over what Mr. Trump could hope to accomplish by announcing that Iran is noncompliant — other than satisfying what appears to be his compulsive urge to spoil President Barack Obama’s legacies. Without proof of Iranian noncompliance, U.S. partners in the nuclear deal, including the European Union, Russia and China, would surely refuse to support the nullification of the accord or the reimposition of sanctions. Iran might respond to decertification by resuming uranium enrichment, even if Mr. Trump did not reimpose U.S. sanctions. That would present the White House with the ugly old problem of how to stop Iranian progress toward a bomb. Could Mr. Trump credibly threaten Iran with military action even while using the threat of force against North Korea?

The principal weakness of the nuclear accord is its temporary nature. Most of its provisions will expire in eight to 13 years, leaving Iran free to stockpile an unlimited quantity of nuclear materials. It follows that the challenge for a rational U.S. administration would be not how to get out of the deal now, but how to extend its restrictions into the future. U.S. partners would likely be ready to cooperate in a strategy aimed at that goal — and they ought to be pressed to do more to stop Iran’s non-nuclear misbehavior. But there is no reason to expect support for a foolish U.S. move that would rekindle a dormant Iranian threat while tolerating its truly dangerous behavior.