They're happening again. In an elegant Vienna hotel, international diplomats and negotiators, including U.S. Secretary of State John F. Kerry, will be locked in discussions with interlocutors from Iran over a deal aimed at settling global fears over Tehran's nuclear program.

Few expect a final agreement to be sealed before an artificial June 30 deadline, but the onus is on American diplomats to reach a final deal by July 9. After that date, a new law passed in Congress giving U.S. lawmakers oversight over a potential deal would mandate a 60-day review period (instead of 30 days), allowing more time for American opponents of rapprochement with Iran to drum up support for their case.

In April, officials from Iran and the P5+1 (the permanent members of the U.N. Security Council, which includes the United States, plus Germany) announced a framework deal in Lausanne, Switzerland, that was considered a significant breakthrough. As WorldViews explained then, its terms would make it incredibly difficult for Iran to move toward building a nuclear weapon without the international community being able to intervene and stop it.

The framework agreement curbs Iran's ability to enrich uranium at levels that would produce enough fissile material for a bomb. Iran also acceded to a reduction of its current number of centrifuges, the whirling devices used to enrich uranium, and to a long-term regime of international inspections. In return, Iran was given assurances of sanctions relief to its crippled economy--Iran's oil exports have been halved since 2012 and the country has been largely cut off from the global financial system.

Despite the considerable wariness of the international community, Iran maintains that its nuclear program is intended for civilian purposes only. At no point was a wholesale dismantling of Iran's nuclear infrastructure -- what some opponents to a deal want to see -- ever in the cards. Nor is a final deal tethered to Tehran's improved behavior as a state actor in the Middle East, where some say Iranian proxies continue to play a destabilizing role.

"I call it the unicorn deal, because it's very pretty but it's ultimately mythical," a senior White House official recently told Bloomberg View. "We have no way to get to that outcome."

But momentum in the talks has stalled since April, and there remain some thorny issues that need ironing out if the Obama administration is to clinch its long-sought prize of a pact with the Islamic Republic.

The main sticking points

Inspections

The April framework deal provided for a tough regime of inspections of Iranian nuclear facilities for more than two decades, including regular access to Iran's nuclear sites, continuous monitoring of Iran's uranium mills and storage facilities, and oversight over the country's remaining centrifuges as well as its procurement of nuclear technologies.

But recent statements from Iran's supreme leader, Ayatollah Ali Khamenei, have raised questions over the inspection terms Tehran is willing to accept. Khamenei signaled, among other red lines, his opposition to inspections of Iranian military sites.

On one hand, it's understandable. No other country willingly grants the world access to its most sensitive military facilities, especially when it's already cooperating with other invasive monitoring protocols. As Fred Kaplan notes in Slate, arms-control treaties between the Soviet Union and the United States never provided for "on-site inspections."

But given the current context, where Iran has to prove its long-term good faith in order to claw itself out of a deepening isolation, Khamenei's current intransigence presents a problem.

Sanctions relief

Meanwhile, Khamenei has also demanded that economic sanctions on Iran imposed by the United Nations and the United States be immediately lifted after the signing of a nuclear agreement. This is a contentious matter, with U.S. proponents of a deal adamant that sanctions relief be phased in over time as Iran adheres to the terms of the agreement.

As my colleague Steve Mufson reports, negotiators are wrangling over a U.N. resolution that would be worded in such a way that Iran can claim it has won sanctions relief even if "the terms of the resolution do not take effect immediately."

Trita Parsi, president of the National Iranian American Council, which is lobbying for passage of the deal, writes that there's a scheme in place to handle the question of when exactly the sanctions are lifted.

More tricky, though, is the Western desire to reimpose sanctions -- or "snap back" -- should it emerge that Iran has not complied or has violated the terms of the final agreement. The trick, one analyst tells Mufson, is to have "layers of consequences for different violations," rather than a rigid, wholesale reimposition of tough sanctions should Iran be found in breach of the agreement.

Critics argue, though, that it will be very difficult to adequately"snap back" sanctions once they've been dropped and after a host of other nations better disposed to Tehran have resumed full trade ties.

Geopolitics

Ultimately, the largest stumbling block to a deal is the gulf of trust between both sides. Hard-liners and hawks in both Washington and Tehran have sought to stall or scupper the talks over the past two years. The government of Israel, a prominent American ally, has noisily agitated against any rapprochement with Tehran. Sunni Arab states in the Persian Gulf region are also wary of a U.S. accord with Iran, the Middle East's preeminent Shiite power.

Proponents of the deal say its terms will be premised not on good faith, but on verifiable measures that could build trust over time. Iran's interests in the Middle East, particularly in combating the jihadists of the Islamic State and stabilizing Afghanistan, increasingly dovetail with those of Washington.

Critics lament that sanctions relief will not be linked to Iran's other questionable activities, including its support of militant groups elsewhere in the Middle East that are considered to be terrorist organizations. But, no matter what deal is reached, other sanctions on Iran's procurement of ballistic missiles and other weaponry will remain in place.

For the past three decades, Iran has had a difficult, acrimonious relationship with the West, and particularly the United States. From hostage crises to terror attacks, the deaths of hundreds of civilians to the legacy of a bloody regional war with Iraq, both sides have their grounds for grievance.

No deal forged in Vienna in the following days will wash away years of skepticism and hostility on its own. And it might not even assuage suspicions over Tehran's actual intentions. But it's a start.

"Fencing in Iran’s nuclear capabilities over the next 20 years is more critical than a full confession of what they did 20 years ago," writes Joe Cirincione, president of the Ploughshares Fund, a global security organization.

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