His decision itself came as little surprise -- the US leader has long scorned what he called the "disastrous" 2015 accord -- but his suggestion that the regime must change underlined the risks of a dangerous new escalation in the Middle East.
US Secretary of State Mike Pompeo promised that Washington will work with its friends to build a new agreement to curtail Iran's alleged quest for nuclear weapons -- but there was no disguising that Trump's decision marked a stark diplomatic defeat for Europe, whose leaders had begged him to think again.
"I am announcing today that the United States will withdraw from the Iran nuclear deal," Trump declared in a White House address, branding the landmark 2015 accord that was endorsed by Britain, China, Germany, Russia and Barack Obama's previous US administration "defective at its core."
Trump -- who enjoys close ties with Iran's foes Saudi Arabia and Israel -- said he had consulted America's friends in the Middle East and concluded "that we cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of the current agreement."
"America will not be held hostage to nuclear blackmail," Trump vowed. "We will not allow American cities to be threatened with destruction and we will not allow a regime that chants 'Death to America' to gain access to the most deadly weapons on Earth."
Trump's hawkish National Security Advisor John Bolton said that European firms would have a "wind down" period to cancel any investments made in Iran under the terms of the accord, after the world agreed to give Tehran sanctions relief in return for it scaling back its enrichment program and placing its nuclear industry under international inspection.
But there was no disguising the dismay in European capitals, whose diplomats see the deal as the best way of keeping tabs on Iran's ambitions while heading off the risk of a destabilizing new arms race in the Middle East, where tensions are already soaring between Tehran and both Israel and the Gulf monarchies.
"France, Germany, and the UK regret the US decision to leave the JCPOA (Iran deal). The nuclear non-proliferation regime is at stake," France's President Emmanuel Macron wrote.
"We will work collectively on a broader framework, covering nuclear activity, the post-2025 period, ballistic activity, and stability in the Middle-East, notably Syria, Yemen, and Iraq," he added.
The European Union's chief diplomat Federica Mogherini, who helped oversee the talks with Iran that led to the Joint Comprehensive Plan of Action, insisted the accord "is delivering on its goal which is guaranteeing that Iran doesn't develop nuclear weapons."
And she added: "the European Union is determined to preserve it."
In contrast, Israel's Prime Minister Benyamin Netanyahu, who last week released a trove of intelligence on a pre-2003 Iranian plan to develop a nuclear weapon which Trump cited approvingly in his speech, was overjoyed.
"Israel fully supports President Trump's bold decision today to reject the disastrous nuclear deal," Netanyahu said, in a televised address, even as his country opened bomb shelters and put the military on high alert in case of attack from the Iranian forces deployed in Syria in defense of Bashar al-Assad's regime.
Saudi Arabian state media said the kingdom "supports and welcomes" Trump's decision.
Iran's President Hassan Rouhani -- who some deal supporters see as a reforming moderate who will be undermined by any collapse in the deal -- was furious, accusing Trump of "psychological warfare" and vowing to take the matter up with the agreement's other signatories, including Washington's rivals Moscow and Beijing.
No 'Plan B'?
Meanwhile, former Obama administration officials warned that the decision puts the US on a collision course with Iran, distances the White House from its key allies and put US citizens held in Iran at risk.
Former deputy secretary of state Tony Blinken, who helped negotiate the accord, called it a "monumental mistake" and former senior diplomat Wendy Sherman told reporters Trump was putting international stability at risk for purely domestic political purposes.
"This has been a crisis that Trump has been precipitating himself to answer his base, to fulfil a campaign pledge that he made, without any sense whatsoever of what Plan B is," she told reporters.
"It says that the United States is not a reliable partner," she added, insisting that the existing deal permanently prevents Iran from gaining a nuclear weapon.
Blinken said ending the deal gives "hardliners in Iran an excuse to restart their pursuit of nuclear weapons, but without the united international coalition to oppose them or inspectors on the ground to expose them."
Trump had had until May 12 to decide whether to continue to waive sanctions on Iran's central bank and its oil sector dealings, a key pillar of the 2015 agreement, but moved more quickly than expected and cancelled sanctions that were not yet up for review.
Fall of regime?
For months, critics have been warning ending the waivers would unravel the carefully constructed deal, plunge Iran's already struggling economy into crisis and expose the biggest transatlantic rift since the Iraq War.
But some US officials close to Trump, as well as hawkish Washington lobbyists, argue that an Iranian economic collapse could lead to Tehran's Islamist regime falling -- and that this would be a good thing.
Ahead of Trump's verdict, diplomats shifted into damage limitation mode, hoping that beyond his inevitably harsh rhetoric, he stops short of immediately reimposing sanctions.
US president Donald Trump announced that US will withdraw from the nuclear deal. Source: AAP
In Brussels, officials are already working on "blocking" measures that would protect EU citizens and companies from US prosecution.
"We are having conversations obviously and we are working on a number of proposals that could protect European companies and operators," a senior EU official told
