Intelligent men and women of good will are lining up on both sides of the fateful choice Congress faces in September: whether to approve or reject the nuclear deal with Iran.

Part of what’s going on is an unfortunate mixing together of two quite different questions, one looking backward and one looking forward.  First, should the Obama administration and other major powers have gotten a better deal?  Second, given the deal the negotiators did produce, whatever its warts, is it better for U.S. and world security to accept it or reject it and try to force Iran to agree to a better one?

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Could the deal have been better?  Certainly.  I wish that the key restrictions on uranium enrichment lasted longer than 10-15 years, that the cuts it imposes on Iranian centrifuges went even further than they do, and that the inspections were even more immediate and far-ranging.  I can imagine that some different combination of offers, threats, and promises, offered by some different set of negotiators, might have produced a better outcome. 

But compromise is the essence of negotiation – and the deal as it stands represents surprising compromises by Iran.  Critics in Tehran are complaining that their negotiators gave up on almost all of the “red lines” that Iranian Supreme Leader Ayatollah Khamenei had said should not be crossed.  I can also imagine that other tactics, rather than leading to a better deal, might have led to a complete breakdown of talks or far less sweeping Iranian concessions.

In any case, whether we could have gotten a better deal is in the rear-view mirror now, and should not be the focus of as much of the debate as it has been.  The real question now – a separate one – is whether the United States would be better off with, or without, the deal as it was negotiated.  The answer to this question seems clear.

With the deal, Iran will take down two-thirds of its uranium enrichment centrifuges, eliminate 98 percent of the enriched uranium stock it has built up (which could otherwise give it a major head start in making material for a bomb), block off the plutonium pathway to the bomb almost completely, and accept and accept a remarkable set of inspections, including the right for inspectors to go to any site they suspect may be carrying out nuclear activities, whether it is a military facility or not.  The most important aspect of the inspections – Iran’s acceptance of the broader inspections called for in the Additional Protocol – would last indefinitely.  There would be less chance that Iran could get to the bomb without being found and stopped, less military threat to Iran, and a flow of benefits into the pockets of some Iran’s most powerful figures that they will be reluctant to have cut off, all of which will undermine the arguments of Iran’s bomb advocates.

But if the United States walks away, Iran will be free to add more centrifuges, enrich more uranium, keep building a reactor well-suited for producing plutonium, test advanced centrifuges whose increased capability would make a covert facility easier to hide, and reject the broader inspections the deal calls for.  And the rejection of the deal would deal a potentially fatal blow to Iran’s advocates of compromise, led by President Hassan Rouhani and Foreign Minister Javad Zarif, confirming the hard-liners’ view that the United States cannot be trusted and emboldening Iranians who argue that rather than negotiating, Iran should move straight to the bomb.

Meanwhile, the global sanctions regime – which countries around the world supported precisely to help get a deal like this one – would likely erode, as the key countries that have upheld the sanctions regime saw  that it was the United States, not Iran, that was walking away from the deal. China, Russia, and others would likely go back to buying Iranian oil, and other sanctions would likely begin to unravel. Iran would get some of the sanctions relief it wants without having to constrain its nuclear program.

Some in Congress argue we could force the rest of the world to go along with our sanctions by threatening to cut companies and countries out of the U.S. market if they dealt with Iran.  But for some of the most important players – such as China, India, or Japan – this is simply not realistic.  China holds over a trillion dollars in U.S. treasuries, and it is the world’s second largest economy; trade with China is as critical to U.S. companies as it is to Chinese companies.

Hence the chances of a better deal down the road seem very slim – and the dangers of trying that path seem very, very high.  Ultimately, faced with an expanding Iranian nuclear program and few sanctions tools or diplomatic options to confront it, the next president could face a very difficult choice between acquiescing in an Iran poised at the edge of a nuclear weapons capability with few inspections in place or launching military strikes.

Moreover, rejecting a deal that had been so painstakingly worked out with all of the world’s major powers and unanimously approved by the UN Security Council would deal a devastating blow to U.S. credibility and that of the Security Council.  That could undermine efforts to resolve many other world issues through diplomacy.

In short, focusing on the question that is really on the table now – to approve or reject the current deal – makes the choices clearer.  The risks of accepting the deal are real, and significant.  But the risks of rejecting it are much, much higher.  The deal offers the best available chance of ending up 10-20 years from now with an Iran that still does not have nuclear weapons.

Bunn is a professor at Harvard Kennedy School’s Belfer Center for Science and International Affairs and is co-principal investigator with the Belfer Center’s Project on Managing the Atom. He is a former adviser on nonproliferation in the White House Office of Science and Technology Policy, where he focused on control of nuclear weapons and materials.