WASHINGTON — Over the strenuous objections of its closest allies, the Trump administration is planning to reimpose United Nations sanctions against Iran on Saturday, though it is unclear how effective they will be without the cooperation of the world’s other major powers.
American officials said the full range of the sanctions would be announced on Saturday evening in Washington. But in the hours beforehand, Britain, France and Germany said in a letter that the sanctions — which the United Nations had suspended after the signing of the 2015 nuclear accord with Iran — would have no legal effect.
To underscore their fundamental opposition, the letter said all three countries would work to preserve the 2015 agreement, which they jointly negotiated with the United States, China and Russia, even as Washington sought to destroy its last remnants. The Trump administration withdrew from the agreement more than two years ago.
“We have worked tirelessly to preserve the nuclear agreement and remain committed to do so,” said the letter, a copy of which was viewed by The New York Times.
For Mr. Trump, the penalties have both a political and international calculus. He ran in 2016 declaring that the Iran deal was a “terrible” giveaway to the country’s leadership, and Saturday’s move will enable him to enter the last stretch of the 2020 election declaring that he had destroyed it, and punished the Iranian economy by resuming sanctions that existed before the Obama administration negotiated the deal.
And should former Vice President Joseph R. Biden Jr. win in November, the resumption of sanctions will make it more complicated to reassemble some version of the agreement. Mr. Biden would have to reverse the move, making it appear he had made a concession to Iran even as it has resumed work on its nuclear program in reaction to Mr. Trump’s decision to abandon the deal.
But for the president’s critics, the move underlines how his administration has splintered alliances and fractured understandings with the United States’ superpower adversaries, Russia and China. They had been unified in reaching the 2015 agreement. Now the United States has gone its own way, and Russia and China seem poised to resume conventional weapons sales to Iran next month when an arms embargo against Iran expires, over the objections of Washington.
“The irony I see here is that Trump is actually doing the U.N. and multilateralism a big favor, because by invoking the snapback, he is putting on display that the kind of clumsy unilateralism that he is known for doesn’t work,” said Ian Johnstone, a professor of international law at the Fletcher School at Tufts University.
He predicted the sanctions would not be enforced by other countries and would be “met with a collective shrug.”
“The U.S. will insist that the sanctions are back on and most other countries will say, ‘No, they’re not,’” said Mr. Johnstone, who advised Kofi Annan, the former United Nations secretary general.
In fact, the Trump administration’s insistence on reimposing the original United Nations sanctions, which had been formulated during the Bush and Obama administrations to force Tehran to the negotiating table, means that the United States alone will enforce them. That has raised doubts over whether the sanctions will bring much, if any, additional economic pain; one Security Council diplomat compared the American saber-rattling to pulling the trigger of an unloaded gun.
Administration officials disagree. “We believe deeply that this is good for the peoples of all nations,” Secretary of State Mike Pompeo told journalists on Wednesday at the State Department.
He said he expected other member states of the United Nations to enforce the sanctions, including by continuing the global arms embargo against Iran that is set to expire on Oct. 18. An effort by the United States to extend that embargo failed in the Security Council; only the Dominican Republic voted with the United States.
“They remain the world’s largest state sponsor of terrorism,’’ Mr. Pompeo said of Iran, “and we don’t believe that them being able to trade in weapons of war with impunity is remotely acceptable.”
In a visit to Washington on Wednesday, the British foreign secretary, Dominic Raab, gamely sought to play down the dispute in his own comments at the State Department.
“There may be shades of difference, but we always manage them as constructively as we have to this point,” said Mr. Raab, standing next to Mr. Pompeo.
In Iran, President Hassan Rouhani declared Saturday and Sunday to be “days of victory” in a thumb to what he described as the toothless American effort to punish his country.
“This is a historic victory for Iran,” he said this week, adding, “The U.S. is isolated and embarrassed.”
The practical effect of the sanctions — beyond the breach of comity of nations — may well be overblown.
The European Union has its own arms embargo against Iran that is not set to expire until 2023, once the International Atomic Energy Agency rules that the government in Tehran has followed the terms of the nuclear deal. The European arms embargo could be lifted sooner, but only if the I.A.E.A. formally certifies that all of Iran’s nuclear activity is peaceful.
That is unlikely, after Israel conducted a raid on an Iranian warehouse two years ago that contained weapons plans dating back two decades. And in reaction to Mr. Trump’s withdrawal from the deal, Iran has already exceeded the limits of nuclear fuel production as defined by the accord with world powers.
Until the European arms embargo is lifted, foreign diplomats predicted that companies and business employees in Europe would continue to adhere to it — making the United Nations sanctions largely irrelevant for Western allies.
Elliott Abrams, the State Department’s special envoy for Iran policy, said the United Nations sanctions should compel European nations to “cooperate closely with us as, and when, they see any effort by Russia, China, or anybody else to sell arms to Iran.”
He said that would also include transfer of nuclear and missile-related technologies to Iran.
But beyond that, the sanctions as described by Mr. Abrams last week would mostly target Iran, banning its nuclear enrichment and reprocessing activities and its ballistic missile program.
It was not clear if the Trump administration would also demand that other United Nations sanctions against Iran — against its oil, minerals and financial sectors — be enforced globally. If not, the effect would fall far short of the sharp economic crackdown that the United States is promoting.
Mr. Abrams said more details would be released starting on Monday. He predicted the return of the sanctions “will have a very significant impact,” given that much of the world’s economy runs through American financial systems and could easily snare violators — at least by stopping the trade.
The economic penalties are being reimposed as debate at the annual United Nations General Assembly begins on Tuesday. The theme of the session this year, the “necessity of multilateralism,” is a pointed message to the United States, which is also pulling back from the World Health Organization and taking punitive action against the International Criminal Court’s prosecution of war crimes in Afghanistan. Both organizations are supported by the United Nations, which itself is a frequent target of criticism from the Trump administration.
Farnaz Fassihi and Rick Gladstone contributed reporting from New York.