As the June 30 nuclear deal deadline draws to a close, the United States, other world powers and Iran are back in talks, and this round might just be the deciding one.
After nearly a decade of international diplomacy, negotiators are working past Tuesday's deadline, trying to reach a final agreement that would curb Iran's nuclear activities for a decade and put tens of billions of dollars back into their economy by easing financial sanctions.
Yet, significant obstacles remain.
Iran says it won't allow inspectors to visit military sites and interview scientists to ensure Iranian compliance. Ayatollah Ali Khamenei, the Iranian supreme leader, also says he wants all economic sanctions lifted before the deal is signed, while the US, Britain, France, China, Germany and Russia must still agree among themselves -- and then with Iran -- on a slower schedule for rolling back sanctions and a plan for snapping them back into place if the Iranians are caught cheating.
Meanwhile, Israel is threatening potential military action to destroy Iran's nuclear infrastructure and Saudi Arabia's Sunni monarchy is considering an atomic programmeme of its own to match that of its Shiite neighbor.
Here is a look at the emerging Iran agreement, based on last public information, remaining obstacles and the political challenges it faces:
What's on the table?
To ease the biggest threat posed by Iran's nuclear programme, negotiators are limiting the number of centrifuges that Iran can install, to a little more than 6,000 for 10 years. Of these, some 5,000 can be in operation; they can only include Iran's basic, least efficient model for enriching uranium.
Uranium can be enriched for energy, medicine and science purposes, as Iran claims are its goals. It can also be spun into material for a nuclear warhead. The deal locks in place restrictions on Iran's enrichment so material stays far below weapons-grade. It also forces Iran to deeply cut its stockpile of enriched Uranium over 15 years.
Similarly, Iran must redesign a nearly built heavy water reactor at its facility in Arak so it can't produce weapons-grade Plutonium. The original core of the reactor will be destroyed or exported. The Iranians can't build another heavy water reactor for 15 years.
The restrictions are more severe at Iran's Fordo enrichment facility, dug deep into a mountainside and possibly impervious to an air attack by the United States or Israel. Here, Iran cannot enrich uranium or conduct uranium-related research and development for at least 15 years as the site becomes a nuclear physics and technology research center. Centrifuges running at Fordo must use other material that cannot be turned into bombs.
The enrichment constraints are designed to extend the time it would take Iran to amass enough material for a bomb if it secretly attempts to develop one. The so-called breakout time for Iran is currently around two to three months. The US says the deal extends the timeline to at least a year in the first decade.
Transparency and sanctions:
The UN's International Atomic Energy Agency will monitor Iran's nuclear facilities and have access to the programme's entire supply chain. Inspectors can examine Uranium mines and mills, and maintain continuous surveillance of Iran's centrifuge rotors and storage facilities for unused machines. Iran must allow the IAEA to investigate suspicious sites or allegations of covert nuclear work. It will work with world powers on a list of actions to help the IAEA resolve decade-old suspicions about past Iranian nuclear weapons work.
US and European nuclear-related sanctions will be suspended after the IAEA verifies Iranian compliance. If Iran violates the deal, sanctions can be re-imposed. UN Security Council resolutions on Iran will be lifted simultaneously with Iran fulfilling commitments related to enrichment, Fordo, Arak and other matters. Disagreements will go to a dispute resolution process.
Areas of Disagreement
Iran is playing hardball on military installations that nations have long suspected of nuclear involvement. Khamenei this week rejected allowing inspections or allowing Iranian scientists to be interviewed. The government enacted legislation this week banning such access. The US has backed off its talk of "snap" or "anywhere, anytime" inspections. But it says a deal hinges on monitors being allowed to investigate what they deem necessary and within a reasonable period of time.
Moreover, Iran wants sanctions lifted up front while the West says Iran must first fulfill its responsibilities. Khamenei says the US approach takes too long and would not include a "complete lifting of sanctions". The Obama administration is hamstrung in how fast it can move because of Congress. It also is struggling to separate sanctions it will suspend in an agreement from others it wants to keep, such as those counteracting Iranian ballistic missile efforts or punishing it for its human rights and terrorism records.
World powers must devise a formula among themselves for quickly reinstating sanctions if the Iranians break the accord. For UN measures, Russia and China traditionally have opposed any plan that would see them lose their veto power.
Another contentious issue is Research and Development. An April, framework between world powers and Iran was vague on permitted levels of research and development. For advanced centrifuges, the US said Iran can engage in "limited" R&D. After 10 years, it said Tehran must adhere to an R&D plan it submits to the IAEA. Khamenei has responded by saying that Iran won't even accept an initial decade of such restrictions, calling the demands "excessive coercion".
The US Congress can weigh in but voting 'no' will not kill the deal as President Barack Obama doesn't need congressional approval for a multinational deal that is not designated a treaty.
However, lawmakers have 30 days to review the agreement, during which Obama can't ease penalties on Iran. If negotiations drag on past July 9 without a deal, that review period extends to 60 days. If lawmakers were to build a veto-proof majority behind new legislation, enacting new sanctions or preventing Obama from suspending existing ones, the administration would be prevented from living up to the accord.
Meanwhile, Iranian hardliners are not backing off either. When Iran's Parliament voted last weekend to ban the access to military sites, some lawmakers chanted "Death to America". The scene underscored the deep opposition to any understanding with a country that hardliners in Iran refer to as the "Great Satan".
Backed by Khamenei, they will examine the deal for any sign of concessions. Groups beyond the governmental control, such as the Revolutionary Guard Corps, may not be keen to implement the requirements.
Apart from the two internal opposition, Israel poses a threat to the deal as well. The Jewish state's leaders have lobbied aggressively against a deal they see as paving the way to a future Iranian nuclear arsenal, which they consider a grave national security threat.
Israel has threatened for years to attack nuclear sites if it feels the Iranians are getting too close to weapons capacity. Prime Minister Benjamin Netanyahu has appealed directly to the US Congress to maintain pressure on Iran through economic sanctions.
Another external factor is Saudi Arabia, a country which claims it will do everything to guarantee its security. Along with the other Sunni monarchies of the Persian Gulf, Saudi Arabia is particularly concerned about Iran recouping up to $100 billion in blocked assets overseas and funneling some of that money into insurgencies and terrorist activity throughout the middle East. The Saudis have been coy on whether they may start a nuclear enrichment programme to match Iran's capabilities in response to an agreement.
The French too have taken a public posture of being even tougher on proliferation than the Americans. They delayed a 2013 interim agreement with Iran out of concern that the deal wasn't tough enough on Tehran. French officials have made similar complaints about the current, emerging package and threatened to block consensus unless their concerns are addressed.