SIBIU, Romania — Tehran’s decision to restart part of the activities prohibited under the landmark 2015 nuclear deal ushers in a new phase in the dangerous escalation triggered by U.S. President Donald Trump exactly one year ago.

The move risks killing an agreement that is crucial not just for the nonproliferation regime and for Middle East security, but also for the future of multilateral diplomacy and the rules-based global order.

If the Joint Comprehensive Plan of Action, as the deal is called, were indeed to collapse, the consequences for regional, European and international security would be dire.

For a year after the U.S. unilaterally withdrew from the JCPOA, notwithstanding Iran's full compliance with it, European leaders have clung to the hope that Iran would stick to its side of the bargain while not receiving its fair due. It was a hope that rested on the conviction that Iran benefits more from remaining in the JCPOA than from following the U.S. drumbeat toward war.

Tehran, for the first time since 1979, found itself with the near-unanimous support of the international community. It was fully abiding by international law and multilateralism. It had the moral high ground, and all blame rested squarely on the U.S.

Europeans have been unable to safeguard their oil imports from Iran and prevent it from being disconnected from international financial markets.

Not only did Iran have the backing of Russia and China, the U.S.’s staunchest allies — the Europeans — were also unequivocal: Iran, as certified repeatedly by the International Atomic Energy Agency, was abiding fully by the deal; the U.S. was not.

True, Iran was losing the economic benefits that it was promised, as companies and banks around the world shied away from doing business with the country for fear of incurring U.S. extraterritorial sanctions.

But European leaders pledged to keep their side of the bargain. Trade with Europe, however small, would still provide the economic advantages needed for Iranian supporters of the JCPOA to win the political argument against those advocating immediate withdrawal.

Sadly, when it came to putting words into action, the European approach was timid. Last summer the European Union passed a revisited blocking regulation promising to prevent European companies from suffering from U.S. extraterritorial sanctions. They approved modest funds to support investment in Iran's ailing economy. And the so-called E3 — France, Germany and the U.K. — declared they would establish a complex financial mechanism to allow trade in humanitarian goods with Iran. Iran's strategic patience, it was hoped, would soon pay off.

Words were not enough. Europeans have been unable to safeguard their oil imports from Iran and prevent it from being disconnected from international financial markets. The E3’s “special purpose vehicle” to facilitate trade in humanitarian goods, called Instex, has yet to become operational. Nor has Iran received much help from China or Russia.

The Iranian public and political class has observed — some in dismay, others with contempt — Europe’s inability to fully live up to the staunch words of support its leaders pronounced.

In the meantime, the Trump administration’s strategy of “maximum pressure” has made life immensely hard for both Iran’s government and the country’s ordinary people. Living standards have fallen significantly due to a combination of reduced government spending, skyrocketing inflation and scarcity of imports (including medicine and food).

The U.S. government claims it wants to change Iran’s behavior, not its regime. But Washington has made demands that, if met, would amount to a full surrender by the Islamic Republic, including ending all work on its nuclear and missile programs and cutting off its support to its allies in Iraq, Syria and Lebanon.

These demands — combined with the recent designation of the Islamic Revolutionary Guards Corps as a terrorist organization (the first time a state institution has been designated as such) and the draconian nature of U.S. sanctions — point in another direction.

The U.S. administration is counting on sanctions being so harsh that social and political unrest would destabilize the regime from within. Only this month, the U.S. refused to extend any waivers from sanctions on Iranian oil imports and has targeted Iran’s metal exports, on which a 600,000 workers-strong industry depends.

The Trump administration has also provoked Iran into abandoning the JCPOA, thereby reopening the possibility of military strikes against its nuclear facilities and air defense systems. This explains why the U.S. has refused to exempt from sanctions even companies whose cooperation is necessary to Iran to comply with the nuclear-related parts of the JCPOA. The intentions behind the U.S. action are not veiled.

The stakes for Europe are sky high. There are no good options, but a decision is needed.

Iran’s leadership has been forced to review its strategic options. The country's hard-line faction argues that Iran should leave not just the JCPOA but also the Nuclear Non-Proliferation Treaty, in order to regain negotiating power vis-à-vis the U.S. But Iranian President Hassan Rouhani, so far, has succeeded in persuading Supreme Leader Ali Khamenei that Iran would lose from such a course of action.

Iran's domestic compromise so far has converged upon reduced implementation of the JCPOA, while remaining within the deal’s legal framework. It has thus announced that it will exceed the limits to its production of heavy water and low enriched uranium if, 60 days from now, the other parties to the JCPOA — the E3, the EU, China and Russia — have not found ways to defend the JCPOA with stronger deeds and not just words.

Iran’s decision puts Europe in a difficult spot. If Iran delivers on its threat and exceeds JCPOA-set limits, the U.S. will push hard to automatically reactivate U.N. and EU sanctions. That would mean the end of the nuclear deal.

The E3 have rejected Iran’s demands as an unacceptable ultimatum. This is the correct decision. But the same determinations should be deployed in defense of the JCPOA. Just as the quest for European autonomy rightly calls upon European leaders not to accept ultimatums, it also requires them not to cave into what has been an outrageous attempt by the U.S. to treat European countries as vassal states and the EU as an adversary.

There are ways to do this, however difficult. Not only should Instex become fully operational, but far more consequentially, the E3 should work with China and Russia to restore at least part of Iran’s ability to export oil, including by envisaging oil swaps between Russia and Iran and using the revenues from those swaps to support the Instex-facilitated trade between European and Iranian companies.

Such a move would create huge tensions with the Trump administration. Yet the rift with Washington could be healed in time. The transatlantic relationship is not only strong enough to withstand the current friction, a healthy transatlantic bond actually requires greater European responsibility in looking after their interests and greater American respect for their European allies and partners.

The alternative, an end of the JCPOA, would gravely damage the global non-proliferation regime and exponentially raise the risk of military escalation in the Middle East. The collapse of the deal would also spell the end of multilateral diplomacy.

The stakes for Europe are sky high. There are no good options, but a decision is needed. European leaders should have the courage to take the right one.

Riccardo Alcaro, coordinator of research at the Istituto Affari Internazionali (IAI), is the author of "Europe and Iran’s Nuclear Crisis" (Palgrave Macmillan 2018). He tweets at @Ric_Alcaro. Nathalie Tocci is director of IAI.