West Asia is in a period of heightened uncertainty. In the Levant, regional powers are scrambling to fill the vacuum created by the steady dismantling of the Islamic State’s sham caliphate across Syria and Iraq. Kurds, buoyed by their pivotal position in this race to Raqqa, have held an independence referendum, drawing the ire of their Iraqi, Turkish and Iranian neighbours, with every chance of a conflagration in disputed, oil-rich areas such as Kirkuk. Turkey continues its authoritarian descent, as its relations with Europe grow sourer by the day. In the Persian Gulf, a crisis within the Gulf Cooperation Council (GCC), pitting Saudi Arabia and the United Arab Emirates against maverick Qatar, has entered its sixth month, with no sign of resolution. Within Saudi Arabia, the young and ambitious heir to the throne, Mohammed bin Salman, is experimenting with an unpredictable mix of reform and repression, with women permitted to drive at the same time as dissident poets, clerics and intellectuals are carted to jail.

However, the biggest shock of all may lie ahead of us. The Joint Comprehensive Plan of Action (JCPOA), a nuclear deal agreed between Iran and six major powers, will celebrate its second anniversary on October 18. It was, and remains, a landmark piece of diplomacy, which recognised Iran’s right to enrich uranium in exchange for a battery of tough, but time-bound, limits on nuclear activity. Through an adroit mixture of pressure, incentives and dogged diplomacy, it defused a crisis that had burned since the 1990s, threatening to spiral into a war in the 2010s. Nevertheless, conservative forces in Israel, the Arab world, and the U.S. denounced the agreement. They complained that it did not address Iran’s non-nuclear behaviour, such as support for Hezbollah and other militant organisations, and that the “sunset” clauses, which progressively relax the constraints on Iran over the next three decades, were too generous.

Last November, one of these critics won the U.S. presidency. In his inaugural speech to the UN General Assembly, Donald Trump called the deal “one of the worst and most one-sided transactions the United States has ever entered into”. Mr. Trump and members of his administration have repeatedly, but falsely, claimed that Iran is violating the agreement. On October 15, he must “certify” Iran’s compliance. If he refuses to do so, it would open the way for the U.S. Congress to re-impose sanctions on Iran, which would automatically violate the agreement.

True to his reality show past, Mr. Trump has declared that he has made his decision, but will not reveal it even to close allies who have asked him, such as British Prime Minister Theresa May.

If Mr. Trump tears up the agreement, all is not necessarily lost. In a recent interview, Iranian Foreign Minister Javad Zarif noted that Europe’s reaction “will have extremely important ramifications for the future of the deal”. The U.K., France, Germany and the European Union have all expressed their categorical support. If the U.S. re-imposes so-called secondary sanctions, which cover foreign companies, Europe would most likely take legal and diplomatic steps to protect its substantial commerce with Iran, even at the cost of a transatlantic crisis.

In the first half of this year alone, EU-Iran trade stood at around $12 billion, a 95% increase over the same period last year. This is roughly thirty times larger than U.S.-Iran trade. European banks, manufacturers and energy companies have also signed dozens of major agreements with Iran over the past year. The EU has jurisdiction over the SWIFT network for cross-border banking transactions. Iran was cut off from this network for four years, but Brussels would resist any U.S. demands to do so again. China, Iran’s main trading partner, and Russia, Iran’s military ally in Syria, would defy U.S. sanctions with even greater enthusiasm. In September, China provided a $10 billion line of credit to Iran’s banks, denominated in euros and yuan, with another $15 in infrastructure projects. In short, it would be virtually impossible to rebuild today the broad, multinational sanctions regime that helped push Iran to the negotiating table during 2013-15. If Iran were therefore persuaded that its re-integration into the world economy could continue regardless, this would be a powerful incentive for Tehran to abide by the JCPOA.

However, a less happy scenario is equally possible. If the deal collapses, Tehran is unlikely to expel inspectors entirely, as Iraq did in 1997, or withdraw from the Nuclear Non-Proliferation Treaty (NPT), like North Korea in 2003. Such steps would undercut Iran’s professions of peaceful intent and cede the moral high ground. Iran would, however, consider re-starting the nuclear build-up that it had halted after an interim deal in November 2013. Prior to this point, Tehran was rapidly accumulating centrifuges and stockpiles of enriched uranium, such that it could “break out” – accumulate enough fissile material for a nuclear device – within a few months, had it chosen to do so. Absent diplomacy, Iran might have shrunk that time to weeks or days, which would have made it hard – perhaps prohibitively so — to detect any Iranian dash to a bomb. If sanctions were the West’s way of pressuring Iran, nuclear build-up was — and could once more be — Iran’s own bargaining chip.

Certainly, Tehran would have to have to balance the advantages of this course against the risks that it would provoke Europeans into siding, reluctantly, with Washington, and that it may push the U.S., Israel, or both, into a preventive war. While President Barack Obama always kept the military option on the table, his threshold for the use of force is likely to have been considerably higher than that of his erratic, impulsive successor. It is not clear how Iran’s segmented leadership – divided between elected president and autocratic Supreme Leader – will weigh these factors, but the probability of an armed conflict would rise sharply if Mr. Trump walked away.

Not only would a war fail to eradicate Iran’s nuclear know-how, it would have far-reaching regional consequences. Iran’s Revolutionary Guards could unleash Shia militia against U.S. troops in Iraq, and expand support to Afghan insurgents just as Mr. Trump’s surge gets underway. Saudi-Iran tensions would spike, and the risks of a U.S.-Russia confrontation in West Asia would jump dramatically. More broadly, abrogation of the JCPOA would be devastating for Washington’s credibility in future diplomacy. All this would be unwelcome news for India. While Indian imports of Iranian oil have been falling regardless, the Chabahar project, scheduled for completion next year, could face fresh obstacles. Iran-Pakistan relations may also shift unpredictably, and in ways that work against Indian interests.

Foreign Minister Sushma Swaraj is likely to have conveyed these concerns to her U.S. counterpart, Rex Tillerson, at their meeting in New York in late September — but Mr. Tillerson, publicly humiliated by his President on Twitter days ago, appears peripheral to American foreign policy. And so we await the judgment of the mad king.

Shashank Joshi is a Senior Research Fellow of the Royal United Services Institute in London