For many in Europe, Trump's decision was no surprise: He has been a longtime critic of the Iran agreement - a signature foreign policy achievement of the Obama Administration - which placed curbs on Iran's nuclear programme in exchange for lifting international sanctions.
Throughout his presidential campaign, he promised to undo the 2015 deal, and today, after months of deliberations, he did, saying he would reinstate nuclear-related economic sanctions against Tehran.
Inspectors from the UN nuclear watchdog agency have said Iran is in compliance with the accord. But Trump has taken issue with provisions that lift some restrictions on Iran's nuclear efforts in years to come, and he never accepted the premise that it was the best near-term strategy to prevent a nuclear-armed Iran.
"At the heart of the Iran deal was a giant fiction that a murderous regime desired only a peaceful nuclear energy programme," he said in televised remarks at the White House.
"We cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of the current agreement," Trump said.
An American exit threatens the entire accord. Tehran was lured to the bargaining table by the prospect of an injection of international investment to buoy its economy. A renewal of sanctions would give the country's leaders little reason to adhere to their part of the deal.
"The pressure is mounting because there are forces in Iran who never liked the agreement and who claim that there is a feeling that the Iranian side delivered and that our side didn't deliver," a senior European diplomat said,.
But after Trump's remarks, the co-signatories of the agreement - France, Britain, Germany, Russia, China, the European Union and Iran - said they would seek to save it.
Iranian President Hassan Rouhani ordered his diplomats to negotiate with their European, Russian and Chinese counterparts, though he threatened that Iran would commence unlimited uranium enrichment if the continued negotiations do not yield results within several weeks.
The Europeans, likewise, insisted on fighting to keep the deal in force.
"We urge the US to ensure that the structures of the JCPOA can remain intact, and to avoid taking action which obstructs its full implementation by all other parties to the deal," Macron, Merkel and May said.
"After engaging with the US Administration in a thorough manner over the past months, we call on the US to do everything possible to preserve the gains for nuclear non-proliferation brought about by the JCPOA, by allowing for a continued enforcement of its main elements."
Federica Mogherini, the EU's foreign policy chief, said that leaders of the multi-state bloc would discuss Trump's decision in the coming hours and days.
She addressed Iran's leaders, saying that they should "not let anyone dismantle the agreement" and that Europeans will "preserve" the nuclear deal and will defend their security and economic interests.
In Moscow, the official reaction to Trump's announcement was more heated.
Russia's Foreign Ministry said that Trump's move was "in gross violation of the norms of international law." The ministry added, however, that it was open to cooperating with the remaining parties to the agreement and would continue to "actively develop" its ties with Iran.
Russia's ambassador to the EU, Vladimir Chizhov, echoed the sentiments of his European counterparts, telling reporters in Brussels that Russia would work with its European partners and others to try to rescue the agreement.
"At this moment it's too soon to talk about concrete parameters, but you can be sure that efforts to preserve that which is called the Joint Comprehensive Plan of Action will continue," Chizhov said, according to Interfax.
Europeans are planning a range of actions that they hope could keep Iran in the agreement.
One might be allowing the European Investment Bank to increase its business in Iran. Another might be fast-tracking the sanctions-compliance rules that make it hard for small and medium-size European companies to do business with Iran.
"We are having conversations, obviously, and we are working on a number of proposals that could protect European companies and operators," the senior European diplomat said.
But a further easing of restrictions by Europe could put companies in a difficult position. Since US banks are such central players in the global financial system, sanctions from Washington could scare off companies that also want to do business with the United States.
Europeans will also have to weigh the prospect of angering Washington if they work too hard to channel investment to Iran in the face of US disapproval.
"There is not much of the deal the Europeans can save as such, unless they would really be ready to go against the United States," said Cornelius Adebahr, an expert on European policy towards Iran at the Carnegie Europe think-tank in Brussels.
"What the Europeans want is to preserve the deal as much as they can," he said, noting the challenges if, for instance, the US imposes secondary sanctions that would potentially target European companies that have sought to do legitimate business in Iran after the passage of the JCPOA.
"The confrontation will now be whether European companies can do business in Iran," he said.
