WASHINGTON -- President Donald Trump has withdrawn the US from a landmark nuclear deal with Iran and five other world powers, isolating the country from its closest European allies, putting the agreement on life support and setting up a new showdown with Tehran over its nuclear work.Following remarks from the White House Diplomatic Room, Trump signed a presidential memorandum that immediately levies the "highest level of economic sanction" on Iran over its nuclear activity– a move that will put the US in violation of core provisions in the agreement, which traded global sanctions relief on Tehran in exchange for temporary caps and dismantling of their nuclear infrastructure.The entire agreement is now imperiled, as European officials expressed dismay when pressed on a path forward, and Iranian leaders promised a "severe response" that might include their own withdrawal from the landmark pact.Iran's President Hassan Rouhani responded in a speech to the nation that Tehran would, for now, remain within the accord, and would negotiate with Russia and China– "the world's two super powers"– on a path forward. But he said that Iran would prepare to resume uranium enrichment as a contingency."Iran is a country that adheres to its commitments," Rouhani said, "and the US is a country that has never adhered to its commitments."Israeli government officials exalted the decision. "Israel thanks President Trump for his courageous decision," Prime Minister Benjamin Netanyahu stated in a speech following Trump's announcement, calling his decision historic."It is clear to me this deal cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of the current agreement– the Iran deal is defective at its core," Trump said in his speech. "I am announcing today that the United States will withdraw from the Iran nuclear deal." "America will not be held hostage by nuclear blackmail," he added.The president said that intelligence published last week by the Israeli government, which revealed an Iranian archive documenting their experimentation with nuclear weapons technology up until 2003, offered "definitive proof" that the premise on which the Iran deal was clinched was "a lie.""The fact is, this was a horrible, one-sided deal that should have never, ever been made," he said. "A constructive deal could easily have been struck at the time. But it wasn't. At the heart of the Iran deal was a giant fiction."Trump emphasized the need for a deal that would address Iran's development of ballistic missiles– the "means of delivering" nuclear warheads; its "totally unacceptable" sunset clauses, which expire caps on Iran's enrichment of fissile material necessary for atomic bombs; and UN inspector access to Iran's military sites, which have historically hosted some of Tehran's most sensitive nuclear weapons activity."The agreement was so poorly negotiated that even if Iran fully complies, the regime could still be on the verge of a nuclear breakout in just a short period of time," Trump said, warning of a nuclear arms race across the Middle East should the deal remain intact: "Everyone would want their weapons ready," he said, "by the time Iran had theirs."Iran has abided by the terms of the deal, formally known as the Joint Comprehensive Plan of Action, since it was first implemented in 2016. But the Trump administration and its fellow critics believe Iran has has done so because the deal structurally favors them: that on the front end, Tehran received a windfall of sanctions relief, and on the back end, the ability to grow the size and efficiency of their nuclear infrastructure.Earlier in the day, his vice president, Mike Pence, briefed congressional Republicans on Trump’s plans. Republican and Democratic leadership alike have lobbied the president against a withdrawal from the deal absent allied support.Indeed, France, Britain and Germany lobbied Trump to remain in the deal to no avail on in recent weeks, and their leaders spoke by phone on Tuesday ahead of Trump's remarks to coordinate their response. One European official described the response they've received from the State Department as the "deafening silence of US diplomats running for cover" from explaining Trump's strategic plan. "He’s prepared to look at discussions on a much broader resolution," John Bolton, the president's national security adviser, told reporters on Tuesday, calling the deal "utterly inadequate" and "a firm statement of American resolve.""We're out of the deal," Bolton said. "We're out of the deal. We're out of the deal."Mike Pompeo, Trump's new secretary of state, said the US would seek to forge a deal with its allies that addresses the totality of its concerns with Iran's behavior– an ambitious deal that, according to the French and British, could have been built upon the existing agreement."As we exit the Iran deal, we will be working with our allies to find a real, comprehensive, and lasting solution to the Iranian threat. We have a shared interest with our allies in Europe and around the world to prevent Iran from ever developing a nuclear weapon. But our effort is broader than just the nuclear threat," Pompeo said. "As we build this global effort, sanctions will go into full effect and will remind the Iranian regime of the diplomatic and economic isolation that results from its reckless and malign activity."The deal allows Iran to refer the US action to the UN Security Council and to the JCPOA Joint Commission– comprised of Russia, China, Iran, Britain, France, Germany, the EU and the US– for review. If neither body can resolve the matter, and if the action is determined to constitute "significant noncompliance," then Iran is freed by the deal to restart its nuclear work "in part or in whole."