It is hard not to be pessimistic about the future of the nuclear agreement that was reached with Iran in 2015. President Barack Obama’s signature international achievement now seems in the distant past after his successor Donald Trump unilaterally abandoned it last year, re-imposed US sanctions on Tehran and made it much tougher for everyone else to keep it alive.

The US will not be taking part when diplomats from China, Russia, France, Germany and Britain meet their Iranian counterparts in Vienna this week. They will be discussing the knotty question of how to respond to Iran’s step-by-step breaches of the restrictions agreed in the Joint Comprehensive Plan of Action. (JCPoA),

Britain, France and Germany have all tried to salvage the landmark pact, under which Iran undertook to curtail its uranium enrichment programme in return for relief from US sanctions. But all three powers have failed to deliver the trade and investment dividends promised under the JCPoA as they have been unable to shield Tehran from the renewed US sanctions that have strangled its vital oil trade.

The announcement by Belgium, Denmark, Finland, the Netherlands, Norway, and Sweden that they have joined the Instex bartering system developed by the EU to sidestep sanctions is expected to make little difference to Iran’s worsening economic crisis.

It was therefore extremely telling when France's foreign minister suggested the other day that Paris is now considering triggering a mechanism that could lead to re-imposed UN sanctions. That was an important signal because President Emmanuel Macron has been trying energetically to salvage the agreement – meeting his Iranian counterpart Hassan Rouhani and trying to persuade him, in vain, to see Trump at the UN general assembly.

The mechanism can be used to refer a dispute to a joint commission comprising Iran, Russia, China, the three European powers, and the European Union and then on to the UN Security Council if that commission cannot resolve it. And if the Security Council does not vote to continue sanctions relief, sanctions that were place under previous UN resolutions would be re-imposed - known as a "snapback". The JCPoA is unlikely to survive that. The agreement, in the words of one expert, is now “a candle that is burning from both ends.”

Iran raised the stakes at the weekend when it warned that if the trigger mechanism was used, it would be forced to reconsider some of its commitments to the International Atomic Energy Agency.

French and other European officials say the recent mass street protests in Iran against fuel price rises, in which Amnesty International says at least 160 protesters were killed, have made it harder to find common ground with the Americans.

US officials insist the unrest has proved that Trump’s policy of “maximum pressure” is working. Supreme Leader Ali Khamenei helpfully declared that the Iranian people had “quelled a broad and dangerous conspiracy that was led by foreign agents.”

Iran’s latest carefully-calibrated step in response to US sanctions was taken in the first week of November: engineers began feeding uranium hexafluoride gas into previously mothballed enrichment centrifuges at the underground Fordow plant south of Tehran. That was consistent with its policy of “strategic patience” but the situation is now more volatile than it was a few months ago. Iran’s next move is likely to be really consequential.

Tehran, like Washington, believes in a policy of “maximum pressure.” That is a fair reading of the incident in September when the Saudi Aramco oil installation at Abqaiq was attacked by missiles and drones. That sophisticated assault was claimed publicly by Houthi rebels in Yemen – but the US, Israel and other western governments have no doubt that it was in fact carried out by Iran, and according to one report, personally approved by Khamenei.

Iran did not fail to notice that despite that attack on a highly sensitive target there was in the end no US military response, reinforcing the long-held view that the Twitterer-in-chief in the Oval Office is shy of any military action in the Middle East.

Seasoned Iran-watchers believe disagreements at the top in Tehran are exaggerated and that for all the talk of divisions between hardliners and moderates there is in fact rare unity when it comes to dealing with Washington: all sides agree that sanctions relief of some kind is a necessary face-saving precursor to further negotiations to address American concerns.

Iran’s calculation may be to try to provoke Trump, by staging another limited military operation in the Gulf or in the Bab al-Mandeb straits, that would force the issue onto his agenda – gambling for a positive outcome. According to this line of thinking the president would be motivated to ease sanctions but crucially widen the terms of the JCPoA by spring or summer next year – in advance of the presidential election in November. That approach assumes that Trump would then claim boastfully to have replaced Obama’s agreement – “the worst deal ever” - with a far better one of his own.

Ian is a former Middle East editor, diplomatic editor and European editor for the Guardian newspaper. In recent years he has reported and commented extensively on the Arab uprisings and their aftermath in Syria, Libya and Egypt, along with frequent visits to Iran, the Gulf and across the MENA region. His latest book, a new history of the Palestine–Israel conflict, was published in 2017 to mark the centenary of the Balfour Declaration and the 50th anniversary of the 1967 war. He has an MA in history and social and political science from the University of Cambridge and a PhD in government from LSE. Ian has written for the Economist, the Washington Post and many other publications, and is a regular commentator on TV and radio on Middle Eastern and international affairs. He wrote the introduction to The Arab Spring: Revolution, Rebellion and a New World Order (Guardian Books, 2012); Israel's Secret Wars (Grove Press, 1991), Zionism and the Arabs, 1936–1939 (Taylor & Francis, 1986, 2015); and contributed to the Encyclopedia of the Modern Middle East and North Africa (Macmillan Library Reference, 2004). His most recent book is Enemies and Neighbours: Arabs and Jews in Palestine and Israel, 1917-2017 (Allen Lane, 2017).