WASHINGTON — President Trump declared on Tuesday that he was pulling out ofthe Iran nuclear deal, unraveling the signature foreign policy achievementof his predecessor, Barack Obama, and isolating the United States among itsWestern allies.

“This was a horrible one-sided deal that should have never, ever beenmade,” Mr. Trump said at the White House in announcing his decision. “Itdidn’t bring calm, it didn’t bring peace, and it never will.”

Mr. Trump’s announcement, while long anticipated and widely telegraphed,plunges America’s relations with European allies into deep uncertainty.They have committed to staying in the deal, raising the prospect of adiplomatic and economic clash as the United States reimposes stringentsanctions on Iran.

It also raises the prospect of increasing tensions with Russia and China,which also are parties to the agreement.