PRESIDENT TRUMP'S advisers barely prevailed on him not to scrap the nuclear agreement with Iran last week, staving off for 120 days a self-inflicted foreign policy disaster. But Mr. Trump did the next-worst thing: He issued an ultimatum to Congress and America's European allies demanding far-reaching changes in the accord, and pledging to terminate it if they do not comply. For a man who often boasts about his dealmaking ability, it was a stunningly graceless move that could trigger an unnecessary crisis four months from now.

As we have frequently pointed out, the nuclear deal is flawed, particularly in sunset provisions that eventually would allow Iran to resume enriching an unlimited amount of uranium — which, in turn, would reduce the time it needed to produce a nuclear weapon from more than a year to a few weeks. But that danger lies years in the future, and in the meantime, international inspectors have repeatedly confirmed that Tehran is abiding by the accord. The most sensible U.S. strategy would be to concentrate on curbing other problematic Iranian behavior — such as its intervention in Syria — while developing a longer-term plan with allies to address the sunset provisions.

Instead, Mr. Trump demanded on Friday that Congress adopt legislation and European governments accept a "new supplemental agreement" that would impose new multilateral sanctions on Iran if it "develops or tests long-range missiles, thwarts inspections, or makes progress toward a nuclear weapon." In essence, Mr. Trump is seeking to unilaterally rewrite the 2015 accord, without bothering to negotiate with Iran. The bullying language he directed at allies such as Britain, France and Germany — claiming that those who refused to accept his terms "will be siding with the Iranian regime's nuclear ambitions" — will only encourage their clearly stated resistance to reopening the accord.

If Mr. Trump delivers on his threat — and he left himself virtually no room not to do so — the big loser will be the United States. European governments, as well as Russia and China, will almost certainly refuse to go along with the reimposition of sanctions. The United States will be isolated from its allies, while the regime of Ayatollah Ali Khamenei will have cause to resume its nuclear activities — a rupture for which Mr. Trump will be blamed.

British Foreign Secretary Boris Johnson rightly asked last week what constructive alternative Mr. Trump had to offer to the continued enforcement of strictures that are preventing Iran from building a bomb. The White House offered none. Instead Mr. Trump appears mindlessly bent on reversing the legacy of President Barack Obama, whatever the consequences.

Some members of Congress, including Senate Foreign Relations Committee Chairman Bob Corker (R-Tenn.) and ranking Democrat Benjamin L. Cardin (Md.), have been making a good-faith effort to save Mr. Trump from this folly. Their hope was to craft legislation that did not violate the original deal and was acceptable to the Europeans. Perhaps such a bill can still be finessed — but, if so, it should contain a provision stripping Mr. Trump of authority to void the accord unilaterally. His reckless attacks on it are endangering vital U.S. interests.

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