ROME — There is no way around it. U.S. President Donald Trump’s decision to pull out of the 2015 Iran nuclear deal may have pleased his political base and delighted the leaders of Israel and Saudi Arabia, but for America’s European allies, it is an utter and unjustified betrayal.

Europe has traditionally sought to maintain a strong transatlantic bond, while promoting of a variety of goals in its surrounding regions, beginning with the Middle East. This has often been a complex balancing act, but one that has paid off.

But when it comes to the Joint Comprehensive Plan of Action (JCPOA) — as the nuclear deal is called — reconciling these two objectives is regrettably no longer possible.

Europeans have much at stake in making sure the deal does not fall apart. If Iran emulates Trump, leaving the deal and restarting their nuclear program, it will be nearly impossible to recreate the double-track approach — sanctions coupled with diplomacy — that allowed the deal to be struck.

In an already incandescent Middle East, Iran’s nuclear advancements would most likely trigger a U.S.-Israeli bombing campaign. Iran, with decades-long experience in “asymmetric warfare,” would mobilize its allies in Iraq, Syria and Lebanon in retaliation. The risk of military incidents involving Russia in Syria would grow exponentially.

Hard-liners in Tehran who argued that the U.S. cannot be trusted feel vindicated and are ready to retake control of Iran’s nuclear policy.

Meanwhile, Iran would continue its nuclear work. Even America’s top brass admit that air strikes would slow down, but not destroy, the country’s nuclear program. And with the Iranian bomb eventually becoming a real prospect, the temptation to emulate it would become irresistible in Riyadh, Ankara and perhaps Cairo. The most conflict-prone region of the world would plunge into a nuclear arms race, and the global nuclear non-proliferation regime would collapse.

Even if these events do not to come to pass and Iran’s regime collapses under the sheer weight of U.S. pressure — a fantastical assumption, but one on which Trump has seemingly based his decision — the effects would be highly destabilizing.

There is simply no chance the regime would go down peacefully, not least because it still commands considerable support, notwithstanding the groundless claims to the contrary by Trump, as well as John Bolton, the unapologetic promoter of the disastrous invasion of Iraq in 2003 and now U.S. national security adviser.

The world has been suffering from the consequences of civil wars in Afghanistan, Iraq, Syria and Libya for years. A civil war in Iran would probably be worse than all others combined, with immense spillover effects both in the Middle East — think only of the Kurdish question — as well as in Europe.

Europe’s strategic interests would be massively affected by such degree of destabilization. Refugee flows would increase exponentially, energy prices would soar, terrorist groups would find an inexhaustible recruitment ground, illicit traffickers would thrive. No wonder the Europeans have reacted to Trump’s decision by restating their commitment to the JCPOA.

Keeping the deal alive is a strategic imperative for Europe. Iran has agreed to consider remaining in the agreement if its other signatories — France, Germany, the U.K. (known as the E3) plus the EU, Russia and China — offset the benefits lost due to U.S. withdrawal.

Europe cannot afford letting this window of opportunity close. Supporters of the deal in Iran, already weakened by the U.S.’s intermittent implementation of the deal since Trump’s inauguration, are now shrinking fast. Hard-liners in Tehran who argued that the U.S. cannot be trusted feel vindicated and are ready to retake control of Iran’s nuclear policy.

Europe — along with Russia and China — should reject false claims that Iran has cheated on the deal, given that eleven rounds of JCPOA-mandated in situ inspections by the International Atomic Energy Agency have not detected any breach.

The Europeans should also do whatever they can to protect European companies wanting to do legitimate business with Iran from the long arm of the U.S. Treasury, and offer Iran a package of measures to partially compensate for the U.S.’s exit.

They should also use the JCPOA as a platform to develop a more constructive albeit critical dialogue with Tehran, not least on the myriad of regional questions — from Syria, Iraq, Lebanon and Yemen to Israel-Palestine — which touch European interests at the core. The first seeds of an E4/EU — France, Germany, Italy, the U.K. and the EU — format with Iran holds promise in this respect.

The hard work of diplomacy lies ahead for Europeans to foster their strategic interests while picking up the pieces of the transatlantic bond.

On the other side of the pond, the EU and its member countries should tirelessly convey the message to the American audience — and specifically to those in the administration, Congress and the media who doubt the wisdom of the president’s decision — that applying sanctions would be a serious threat to European security. They should ask for special concessions for their companies and be ready to take proportionate retaliatory measures if denied.

The result no doubt will be an uncomfortable transatlantic divergence. But the transatlantic partnership has lived through many ups and downs before, and the two sides of the Atlantic have always found a way out of them.

Faced with the unfortunate choice forced upon Europe by Trump — between defending the multilateral rules-based order and siding with the U.S. — the Continent’s leaders have rightly opted for the former.

The hard work of diplomacy lies ahead for Europeans to foster their strategic interests while picking up the pieces of the transatlantic bond.

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 and special adviser to European High Commissioner for Foreign Affairs Federica Mogherini.