Mr. Trump, in declaring his intention not to certify Iranâs compliance with the deal, essentially kicked to Congress a decision about whether to reimpose sanctions on Iran and blow up the agreement. His aides, however, insisted that was not the goal, and that they instead wanted Congress to enact legislation defining what would incite the United States to reimpose sanctions.
The president listed three such triggers â the deployment of an intercontinental ballistic missile by Iran, Iranâs refusal to negotiate an extension of the dealâs existing constraint on its nuclear activities and evidence that Iran could manufacture a bomb in less than 12 months. Any of those could prompt the United States to walk away from the deal.
âIn the event we are not able to reach a solution working with Congress and our allies,â he said, âthen the agreement will be terminated.â
Persuading the allies to renegotiate the deal is a far-fetched goal. The leaders of Britain, France and Germany quickly issued a joint statement urging the United States to adhere to the agreement, which they hailed as âthe culmination of 13 years of diplomacy.â Iranâs president, Hassan Rouhani, said his country would consider âno amendment whatsoeverâ to the deal.
And enacting new legislation on Iran would require 60 votes in the Senate, meaning Republicans would need to pick up the support of at least eight Democrats. Senate Democrats had urged Mr. Trump not to withhold certification, and they are unlikely to agree to legislation without assurances that the deal will remain intact.
Mr. Trumpâs scalding critique of the nuclear deal as âone of the worst and most one-sided transactions the United States has ever entered intoâ echoed the language he used during his presidential campaign. But he also acknowledged the obstacles to ripping it up.
âWhatâs done is done,â Mr. Trump said, âand thatâs why we are where we are.â
The president seemed determined to erase any residual hope that the nuclear deal might form the basis of a new relationship between the United States and Iran. His speech, from the Diplomatic Reception Room of the White House, was perhaps the most hostile of any American leader toward Iran since President George W. Bush placed the country on his âaxis of evilâ in 2002.
Mr. Trump recited a litany of misdeeds by Iran going back to the 1979 hostage crisis and described it as the worldâs leading state sponsor of terrorism, supporting Al Qaeda, Hamas, Hezbollah and the Taliban. He also accused the country of dealing with North Korea, a reference to Pyongyangâs long history of selling missile technology to Iran, and said he had asked the nationâs intelligence agencies to investigate whether the relationship went further.
His tone made clear that Mr. Trump has no interest in what, for the Obama administration, was the biggest gamble of the accord: to provide the basis for two longtime adversaries to find other ways to cooperate. He also suggested that Iran would never change.
âGiven the regimeâs murderous past and present, we should not take lightly its sinister vision for the future,â the president said. âThe regimeâs two favorite chants are âDeath to Americaâ and âDeath to Israel.ââ
The nuclear deal is the latest international agreement that Mr. Trump has tried to exit, amend or water down, including the Paris climate accord and the Trans-Pacific Partnership. The closest analogy to this deal may be Nafta, the trade agreement that Mr. Trump once threatened to rip up and is now undergoing a painstaking renegotiation.
Critics said Mr. Trump risked isolating the United States diplomatically and giving up the dealâs hard-won gains, including intrusive inspections of Iranâs nuclear facilities. The angriest voice belonged to former Secretary of State John Kerry, who spent several years negotiating the accord that Mr. Trump denounced.
âIt is very, very poor, nonstrategic diplomacy,â Mr. Kerry said in an interview, his voice rising. Pointing out that âyou cannot unilaterally reset the terms of the deal,â he said the agreement âgives us a quarter-century of absolute accountabilityâ and assured that âthe minute we see the stockpile going up, the questions and red flags will go up like crazy. And 15 or 25 years from now, we still have the same military options we have today.â
âIf you want to have your war, Donald Trump,â Mr. Kerry said, âyou can have it in 20 years.â
Congress is deeply divided on the Iran deal, and getting it to agree on additional legislation could prove difficult. While some Republicans are eager to undermine the deal, Democrats are equally determined to preserve what they view as another legacy of the Obama administration that Mr. Trump is trying to dismantle.
âWe will not buy into the false premise that it is Congressâs role to legislate solutions to problems of his own making,â said Senator Benjamin L. Cardin of Maryland, the top Democrat on the Foreign Relations Committee.
Senator Bob Corker of Tennessee, the chairman of the committee, released a potential blueprint for imposing an automatic return of sanctions if Iran was believed to be capable of producing a nuclear weapon within a year, or if it violated other restrictions.
He worked on the proposal with White House officials and Senator Tom Cotton, the Arkansas Republican who is a hard-liner on Iran, and predicted it could win bipartisan support. It suggests that Mr. Corkerâs bitter personal feud with Mr. Trump will not obstruct cooperation on this issue.
Mr. Corker, acknowledging he had little choice but to âdeal the circumstances as they are,â signaled that he would work to try to find points of compromise â possibly including assurance that the deal would not simply be ripped up by the president. âYouâre going to see all of this evolve in daylight,â he told reporters.
Decertification does not, by itself, affect the American obligation to adhere to the deal, including the lifting of some economic sanctions.
Since becoming president, Mr. Trump has twice reluctantly certified the agreement. But administration officials concluded that he could not bring himself to do that every 90 days, even if the judgment of nuclear inspectors and his own intelligence agencies was that Iran was in compliance.
So they tried to find a solution that would allow Mr. Trump to signal his disapproval of the deal without putting the United States in the position of being the first signatory to violate it. That solution was to declare that the suspension of sanctions was not âappropriate and proportionateâ to the steps that Iran has taken to end its illicit nuclear activities.
That formulation is narrower than declaring that the agreement is no longer in the nationâs national security interests, and administration officials said it was chosen carefully to avoid putting Mr. Trump in a box the next time he had to decide whether to waive sanctions against Iran.
While Mr. Trumpâs speech was described beforehand by his aides as a description of a broad new initiative to push back on Iran, he gave little description of what that would be. Mr. Trump did say the United States would increase pressure on the Islamic Revolutionary Guard Corps, which he described as the âsupreme leaderâs corrupt personal terror force and militia.â
The Treasury Department announced it would designate the Revolutionary Guards as a terrorist group, expanding an existing designation of the Quds Force, the corpsâ paramilitary wing. But Mr. Trump stopped short of putting it on the State Departmentâs list of foreign terrorist organizations.
Such a designation, Mr. Tillerson said, would impede military operations in which American and Iranian forces found themselves on the same battlefield â presumably fighting the Islamic State.