President Trump declared on Tuesday that he was pulling out of the Iran nuclear deal, unraveling the signature foreign policy achievement of his predecessor, Barack Obama, and isolating the United States among its Western allies.

“This was a horrible one-sided deal that should have never, ever been made,” Mr. Trump said at the White House in announcing his decision. “It didn’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 a diplomatic and economic clash as the United States reimposes stringent sanctions on Iran.

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

In a joint statement, President Emmanuel Macron of France, Prime Minister Theresa May of Britain and Chancellor Angela Merkel of Germany urged Iran to “continue to meet its own obligations under the deal,” despite the American withdrawal.
 

“We encourage Iran to show restraint in response to the decision by the U.S.,” the statement said. Separately, in a post on Twitter, Mr. Macron said the European allies “regret” Mr. Trump’s decision, adding, “The international regime against nuclear proliferation is at stake.”

One person familiar with negotiations to keep the accord in place said the talks collapsed over Mr. Trump’s insistence that sharp limits be kept on Iran’s nuclear fuel production after 2030. The deal currently lifts those limits.

As a result, the United States is now preparing to reinstate all sanctions it had waived as part of the nuclear accord — and impose additional economic penalties as well, according to another person briefed on Mr. Trump’s decision.

The withdrawal fulfills one of Mr. Trump’s oft-repeated campaign promises, and came despite intense personal lobbying by European leaders and frantic attempts to craft fixes to the deal that would satisfy him. In part, Mr. Trump was driven by the conviction that taking a tough line with Iran would help an upcoming negotiation with the North Korean leader, Kim Jong-un, whom he plans to meet in the next several weeks.