By Barbara Slavin

With the arrival of Secretary of State John Kerry and his Iranian counterpart, Javad Zarif, in Vienna Friday, the climax to nearly two years of intensive negotiations is at hand.

Opponents and advocates of a long-term nuclear agreement with Iran are marshaling their best arguments in an effort to influence the talks before a June 30 deadline, which a senior State Department official acknowledges may be pushed a few days.

At a Senate Foreign Relations Committee hearing Thursday, two witnesses – David Albright of the Institute for Science and International Security and Ray Takeyh of the Council on Foreign Relations – highlighted what they saw as a major flaw: that key provisions of the impending deal would expire in 10 years. Iran, Albright implied, would immediately ramp up its uranium enrichment program and race to a bomb before the international community could stop it.

A third witness, Jim Walsh, an arms control expert from MIT, provided the counterargument: ten years is a “long time” for an agreement that he said was the toughest ever negotiated in terms of anticipated provisions for monitoring and verification. Iran already has the ability to make a bomb if it wants to, Walsh said, and the decision depends on the government’s political will, which can be influenced by incentives as well as threats of resumed economic punishment.

In an optimal world, a longer-term deal with Iran would be preferable. But a decade-long insurance policy against an Iranian nuclear weapon is surely better than none. At present, Iran could amass sufficient fissile material for a weapon in three months, while the agreement being negotiated would extend the so-called breakout time to one year.

According to parameters that were reached in Switzerland in April and form the basis for what is being finalized now in Vienna, Iran will take a series of nonproliferation steps that would be difficult to quickly reverse. Among them: reducing its stockpile of low-enriched uranium to 300 kilograms – too little to form the core of a nuclear weapon if further enriched to weapons grade – and rendering an underground facility at Fordow unusable for uranium enrichment.

Iran’s commitment not to build bombs stands as long as it remains a member of the nuclear Non-Proliferation Treaty, whose Additional Protocol it will be obliged to ratify, allowing intrusive inspections of non-declared but suspicious sites. A commission to resolve disputes will be created on which the Western members of the so-called P5+1 – the U.S., Britain, France and Germany – will outnumber Iran, China and Russia.

Foreign countries are also expected to provide Iran with expertise and material for its civilian nuclear program that will enmesh Iran further into the international nonproliferation regime and make a dash to a bomb even more unpalatable.

Some advocacy groups that have opposed an Iran deal all along are still raising “poison pill” demands that Iranian negotiators clearly will not accept – such as an insistence by the American Israel Public Affairs Committee that Iran dismantle its entire nuclear infrastructure.

More constructive advice has come from a group of former Obama administration advisers. Their requirements for a good deal include “timely and effective access” to any sites suspected of illicit nuclear activity, limits on Iran’s research and development of more advanced centrifuges and so-called “snap-back” provisions that would allow economic sanctions to be restored if Iran violates its commitments. U.S. officials say all these provisions will be part of the final agreement.

Concerns about whether a good deal is achievable rose earlier this week when Iran’s Supreme Leader, Ayatollah Ali Khamenei, made a speech demanding immediate lifting of sanctions against Iran when a deal is signed and rejecting limits on centrifuge R&D. But other apparent Khamenei “red lines” have faded in the past and his remarks appear aimed at bolstering Iranian negotiating leverage in the final stretch.

In fact, leverage on both sides is limited.

Iran sorely needs the economic shot in the arm that sanctions relief will bring and its negotiators will have trouble walking away from talks in which they have invested so much political capital. At the same time, Iran’s interlocutors understand that the sanctions that helped bring Iran to the table will inevitably erode as foreign companies seek to return to a resource-rich country of 80 million educated consumers.

As the author Peter Beinart noted Thursday at a panel on Iran in Washington organized by the National Iranian American Council, the U.S. cannot unilaterally dictate the terms of an agreement with Iran.  Americans are no longer “as omnipotent as we were” in the days after the collapse of the Soviet Union, Beinart said. The U.S. has “to come to terms about how much power we really have” to dictate the course of events in a world of other rising powers and in a region that is falling apart.

Indeed, Iran is among the most coherent nation states  in a Middle East beset by terrorism, civil war and authoritarian governance.

There is no guarantee that a nuclear agreement will ameliorate Iran’s other considerable shortcomings, including a poor human rights record, backing the Assad regime in Syria, US-designated terrorist organizations such as Hezbollah and regional adventurism. But the absence of a deal is unlikely to influence the Islamic Republic in positive ways.

So the time has come to blot out the noise outside the negotiating room and complete a historic arms control agreement.  A decade without another nuclear weapons state in the Middle East is surely better than the alternative.