The outlines of a potential agreement between the U.S. and Iran are emerging from the indirect negotiations in Vienna. The two sides will probably succeed in their professed aim of resurrecting the nuclear deal secured by President Barack Obama and renounced by President Donald Trump. But how much that would practically accomplish is questionable.

Washington and Tehran say they’re prepared to return, on a “compliance for compliance” basis, to the terms of the Joint Comprehensive Plan of Action they agreed, with other world powers, in 2015. On the American side, that would mean the gradual lifting of economic sanctions on the Islamic Republic; on Iran’s part, it would involve scaling down uranium-enrichment activities and disposing of new stockpiles.

But turning the clock back is more complicated than it sounds, for both technical and political reasons.

The Biden team looks willing to drop nuclear-related sanctions while retaining others based on Iran’s support for terrorism, its human-rights violations and other malign activities — but not all of the sanctions fit neatly into these silos. After taking the U.S. out of the JCPOA in 2018, the Trump administration imposed hundreds of new sanctions on Iran, many of which straddle these categories. It also made many older and re-imposed restrictions harder to revoke.

On the other side of the equation, even if Iran does get rid of its new stockpiles and downgrade its enrichment activities, its nuclear threat will not recede to the level achieved by the 2015 deal. That’s because, having breached the JCPOA’s terms, Tehran has made major strides in mastering sophisticated enrichment technology. Such knowledge, once acquired, can’t be unlearned.

This technological progress has significantly reduced the “breakout time” it would take Iran to manufacture a nuclear weapon, by some estimations to a mere six months.

This is crucial because the nuclear deal was essentially a chronological gamble. In 2015, the Obama administration wagered that 10-15 years of nuclear inactivity would facilitate political changes within Iran that would eventually help extend or improve the arrangement. Now that five years have passed and Iran has reduced the breakout time, those odds have obviously changed.

The politics have changed, too — and, for the American perspective, for the worse. Hard-liners have consolidated their control of most of the levers of the Islamic Republic, and expect one of their own to win the presidential election in June. They may allow the lame-duck President Hassan Rouhani to secure a general understanding with the U.S. and other world powers, but would prefer to delay a full return to the JCPOA until after their man has been elected, so they can claim the credit for the economic benefits accruing from sanctions relief.

To that end, they are putting political hurdles in Rouhani’s path. They have attacked him for suggesting a phased return to the JCPOA terms, and have used their control of parliament to enact legislation narrowing his negotiating options. Rouhani has openly accused his domestic opponents of trying to sabotage his efforts to have the sanctions lifted.

What can the Biden administration do about any of this? In order to get around Trump’s “poison pill” sanctions, the U.S. is offering Iran a combination of formal and informal measures, including discretionary nonenforcement. These measures would in effect restore the status quo that existed between the signing of the 2015 agreement and Trump’s arrival in the White House. But other sanctions on terrorism and human rights issues will remain in place.

Such an arrangement may suit Rouhani’s successor, who would cash in on the economic dividend. The hard-liners will also be reassured by the fact that the Biden team has dropped its demand that Tehran commit to additional talks to extend the restrictions in the JCPOA and reckon with Iran’s missile program as well as its support for a range of violent, extremist militias in the Arab world.

This may get the JCPOA going again, but time and technology have greatly diminished the deal’s value. The challenge for the U.S. will be to achieve what Secretary of State Antony Blinken has described as a “longer and stronger agreement” with the Islamic Republic. For Biden, that promises to be a much higher mountain to climb.

Hussein Ibish is a senior resident scholar at the Arab Gulf States Institute in Washington.

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