European leaders are determined to try to salvage the Iran nuclear deal even though this potentially puts them on a collision course with an uncompromising US president determined to confront Iran as the “leading state sponsor of terror”.

The clash represents a huge test of the durability of the surprisingly concerted alliance that Germany, France and the UK have managed to maintain in their humiliatingly fruitless bid to prevent Donald Tump from explicitly withdrawing from the deal signed by his predecessor Barack Obama.

The risk is that the unity forged by the European trio over the need to preserve the deal now falters as disagreements surface on how far they are prepared to antagonise a determined US president, not to mention Israel and Saudi Arabia, to keep the deal alive.

Q&A What is the Iran nuclear deal? Show Hide In July 2015, Iran and a six-nation negotiating group reached a landmark agreement known as the Joint Comprehensive Plan of Action that ended a 12-year deadlock over Tehran’s nuclear programme. The deal, struck in Vienna after nearly two years of intensive talks, limited the Iranian programme, to reassure the rest of the world that it cannot develop nuclear weapons, in return for sanctions relief.

At its core, the JCPOA is a straightforward bargain: Iran’s acceptance of strict limits on its nuclear programme in return for an escape from the sanctions that grew up around its economy over a decade prior to the accord. Under the deal, Iran unplugged two-thirds of its centrifuges, shipped out 98% of its enriched uranium and filled its plutonium production reactor with concrete. Tehran also accepted extensive monitoring by the International Atomic Energy Agency (IAEA), which has verified 10 times since the agreement, and as recently as February, that Tehran has complied with its terms. In return, all nuclear-related sanctions were lifted in January 2016, reconnecting Iran to global markets. The six major powers involved in the nuclear talks with Iran were in a group known as the P5+1: the UN security council’s five permanent members – China, France, Russia, the UK and the US – and Germany. The nuclear deal is also enshrined in a UN security council resolution that incorporated it into international law. The 15 members of the council at the time unanimously endorsed the agreement. On 8 May 2018, US president Donald Trump pulled his country out of the deal. Iran announced its partial withdrawal from the nuclear deal a year later. Saeed Kamali Dehghan, Iran correspondent

The tone in which figures such as the French president Emmanuel Macron challenge Trump’s judgment call, and bellicose rhetoric, will also be critical to future transatlantic relations. In making his announcement Trump did not hold back, or make any concessions to European sentiment, effectively accusing his European allies of being duped by “a giant fiction perpetrated by a murderous regime”. Still worse, from the EU perspective, he said he was imposing “the highest level of sanctions”, adding explicitly that other nations could be sanctioned if they assisted Iran.

In the face of this rhetoric, Europe’s room for manoeuvre and apparent traction with Washington is limited.

Tony Blinken, a former Obama deputy secretary of state involved in the negotiation of the original deal known as the joint comprehensive plan of action (JCPOA), said Europe’s ability to keep the deal alive would largely depend on whether Iran continues to reap economic benefits, even without the US. “That will be a judgment over time,” he said. “It depends on how companies react to the new environment and it depends on whether the US tries to sanction companies that trade with Iran.”

But the viability of Europe’s plans to keep Iran committed to the deal will depend on how aggressively the US Treasury ensures that any economic sanctions it now imposes on US firms that continue to trade with the central bank of Iran also impact on European firms. The first sanction that is being reimposed by Trump, as a result of ending the waiver, is a requirement for firms to show they are significantly reducing the number of oil deals they are striking with Iran via the country’s central bank. That will take as long as 180 days to measure. But in an uncompromising mood, Trump implied other much wider sanctions will also be reimposed, even though he gave no timeframe for doing so.

One difficulty is that many existing US sanctions on Iran, some linked to Tehran’s human rights abuses and its ballistic missile programme, have already had a chilling effect on risk-averse European banks with commercial links to the US.

Previous efforts in 2016 and 2017 to find a way to disentangle European firms trading with Iran from the threat of US sanctions largely failed, leading Tehran to complain for the past three years that the supposed chief upside– increased western investment in the Iranian economy – had not materialised in contravention of the nuclear deal.

Play Video 2:11 What is the Iran nuclear deal? – video

The question for Europe is whether it can find a more effective means of protecting European business trading with Iran from US sanctions. Blinken pointed out that in the past the EU has staged confrontations with the US when Washington has imposed secondary sanctions. The Libya and Iran sanctions act passed by Congress in 1996 provoked the EU to pass a blocking statute asserting US secondary sanctions on European firms trading with Iran or Libya had no legal effect. The EU also threatened to take the US to the World Trade Organisation. “Broadly the US backed down”, Blinken said.

But Trump seems prepared to hit European firms trading with Iran. The tone of his White House address suggested he will not tolerate any threat to his political authority to settle the west’s relations with Iran.

Nevertheless, over the past few weeks, the EU has been privately reviewing such past confrontations with the US over sanctions, but it has not spoken of its contingency plans in public preferring to focus on how to persuade the US to pull back.

Maja Kocijančič, the EU spokeswoman for foreign affairs, simply said on Tuesday: “We are working on plans to protect the interests of European companies.”

But in a bid to reassure the Iranians, and prevent an uncontrolled escalation, officials from the UK, France, Germany and the EU’s foreign policy service stressed their support for the Iran deal when they met Iran’s deputy foreign minister, Abbas Araghchi for talks in Brussels on Tuesday, hours before Trump’s statement. Europe would implement the JCPOA as long as Iran complies with its obligations, said a German foreign ministry source.

The Iran deal explained: what is it and why does Trump want to scrap it? Read more

At the same time, Europe instinctually will not want to find itself at odds with its natural allies Saudi Arabia, Israel and Washington on such a critical judgment call as the role of Iran in the Middle East. A public confrontation between the US and Europe at the United Nations security council over the future of the agreement is only likely to benefit Russia.

Instead, the EU will have to decide whether to pursue its proposals, originally concocted to persuade Trump to stay in the deal, to negotiate a supplementary agreement with Iran covering its ballistic missile program, its foreign policy interventionism in the Middle East and the controversial sunset clauses in the deal.

At the moment European leaders face only an invidious choice: succumb to Trump’s leadership even though they say it risks turmoil in the Middle East, or challenge their closest ally on probably the biggest foreign policy decision of his presidency.