If finalised by its 30 June deadline and implemented, the nuclear accord could put an end to a prolonged and multidimensional standoff; effectively block overt and clandestine pathways to nuclear militarisation; set a positive precedent for the non-proliferation regime; provide the Iranian people with economic relief; offer a path for normalising Iran’s relationship with the international community; and thus open the door to the possibility of constructive engagement on critical issues of peace and security in the Middle East.

Negotiated outcomes by nature are imperfect. These agreed upon parameters provide Iran with an enrichment capacity higher than the U.S. and its allies preferred, and sanctions relief slower and more circumscribed than Iran desired. But both sides have protected their core interests and rightfully can claim victory – a precondition for any sustainable solution.

This accomplishment is not final; it is as fragile as the forces against it are formidable. It has serious critics in Iran, the U.S. and the region. The most important wildcard are members of the U.S. Congress who maintain that it does not sufficiently restrain Tehran’s nuclear activities. Passing new legislation to thwart this agreement – even though President Obama has committed to veto it – would hinder diplomatic momentum and harm the U.S.’s global standing.

Some fear that the deal will embolden Iran to pursue regional hegemony. But a continuation of the status quo – and more so its deterioration – will not prompt Iranian moderation. Should the negotiations collapse, Iran will accelerate its nuclear program just as the West will step up sanctions. This would lead to a new escalation of tensions in the region and possibly trigger a military confrontation. The agreement’s opponents offer no realistic alternative: there is no better deal attainable, and to hold out for one is to squander this precious opportunity.

It is equally important not to lose sight of regional tensions but to parley this agreement’s positive momentum toward calming them. Both sides should avoid the temptation to take provocative measures against the other in order to demonstrate to their domestic hardliners that nothing else has changed or can change between them. In the run-up to the 30 June deadline and well beyond it, the West should – in cooperation with other regional partners – engage Iran in dialogue over issues of common interest, such as stability in Afghanistan and Iraq and ending the bloodshed in Syria and Yemen. For its part, Tehran should take concrete steps to convince its neighbours that, even as it rehabilitates itself politically and economically, the accord has not come at their expense.

This agreement could turn out to be the most important diplomatic accomplishment in a generation. To secure and sustain it, the parties will have to display the same dedication and discipline that brought them this far.

Open Letter to Iran’s and the P5+1/EU3+3’s Nuclear Negotiators

We would like to recognise the nuclear negotiators representing China, the European Union, France, Germany, Iran, Russia, the United Kingdom, and the United States for their unwavering endeavours since October 2013, which have led to agreement on the framework for a Joint Comprehensive Plan of Action, announced on 2 April 2015.

As veteran practitioners of statecraft who understand the challenges of principled, patient and persistent diplomacy under tremendous domestic and external constraints, we applaud the negotiators for their courage, resolve and flexibility. Solving seemingly intractable international standoffs through peaceful means is a rarity in our troubled times; solutions that provide for win-win outcomes are even rarer.

Like all negotiated solutions, this understanding may not satisfy all stakeholders, but its realistic alternatives – a cycle of mutual escalation leading to an Iranian bomb or bombing Iran – will not stand them in better stead. Nor will this compromise assuredly reduce regional tensions; but its absence would likely further enflame them.

This understanding nonetheless stands as a singular accomplishment of our time, negotiated in good faith, with both sides achieving the maximum attainable under the circumstances. To ensure that this laudable step will lead to a lasting accord, we urge the negotiators to preserve the momentum and promptly finalise the remaining details – as well as critics to give them a chance.

Samuel Berger Former U.S. National Security Adviser, member of International Crisis Group’s Board of Trustees.

Carl Bildt Former Prime Minister and Foreign Minister of Sweden, member of International Crisis Group’s Board of Trustees.

Emma Bonino Former Foreign Minister of Italy, member of International Crisis Group’s Board of Trustees.

Micheline Calmy-Rey Former President and Foreign Minister of the Swiss Confederation, member of International Crisis Group’s Board of Trustees.

Jean-Marie Guéhenno President and CEO of International Crisis Group, former Deputy Joint Special Envoy of the UN and the League of Arab States on Syria, former UN Under Secretary-General for peacekeeping operations.

Thomas R. Pickering Former U.S. Undersecretary of State, Ambassador to the UN, Russia, India, Israel, Jordan, El Salvador and Nigeria, member of International Crisis Group’s Board of Trustees.

Javier Solana Former EU High Representative for the Common Foreign and Security Policy and Secretary General of the Council of the EU, former Secretary General of NATO, member of International Crisis Group’s Board of Trustees.

The Essential Balances in Iran's Nuclear Breakthrough