Negotiations over Iran's nuclear programme will resume this week against a backdrop of tough talk from leaders in Tehran and Washington, which has complicated the already-difficult task of resolving a decade-old row over uranium enrichment.
Negotiators will meet in Vienna on Tuesday, February 18, four weeks after Iran began suspending high-level uranium enrichment in exchange for temporary sanctions relief, the result of a landmark interim deal struck at the end of November.
During that time, domestic tensions have risen in Iran and the United States. Hard-liners in each country have criticised their leaders for weakness by making too many concessions while hammering out deals with a long-standing enemy.
US President Barack Obama has been attacked for appeasing Iran, leaving a key US ally, Israel, exposed - and for nudging Saudi Arabia and other Middle Eastern countries towards countering a potential Iranian threat by hatching their own nuclear plans.
He struggled to dissuade US senators from derailing talks by seeking new anti-Iran sanctions in Congress. Obama has since used tougher language on Tehran, saying it has little wiggle room in proving that its nuclear work is peaceful. "If they meet what technically gives us those assurances, then there is a deal to be made," he said during a While House press conference alongside his French counterpart, Francois Hollande. "If they don't, there isn't."
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Some 10,000km away in Tehran, Iranian President Hassan Rouhani has his own problems. Critics handed out leaflets during celebrations of the 35th anniversary of the Islamic Revolution on February 11, warning that sanctions would continue to hobble Iran even if it dropped its nuclear ambitions.
Rouhani's promise that Iran would pursue peaceful atomic research "forever" - together with news of the test launch of domestic-built long-range ballistic missiles with radar-evading capabilities - was widely seen as a sop to his critics.
"The biggest danger remains attacks from hardliners in the US and Iran," said Meir Javedanfar, a politics lecturer at Israel's Interdisciplinary Center Herzliya. "In Iran, for now, it seems that Rouhani has the support of Ayatollah Khamenei. This is likely to give him room for manoeuvre. However, such support is not open ended, nor is it a blank cheque. Rouhani has to ensure that he always operates within the red-lines set by Iran's most powerful man."
These pressures come as Iran joins the permanent members of the UN Security Council - Russia, France, Britain, the US and China - as well as Germany (the so-called "P5+1" group) in Austria's capital for the next stage of a reconciliation drive that is only expected to get harder.
The interim deal, which took effect on January 20, saw Iran agree to a six-month curb on uranium enrichment and to avoid fuelling or commissioning a heavy-water reactor in Arak. In return, the P5+1 and the European Union provided sanctions relief valued at $6-7bn.
To succeed, this week's talks must deliver a long-term solution by July 20. It must permit Iran's stated desire of using nuclear know-how peacefully while offering safeguards to satisfy the West that Tehran will not develop atomic weapons.
Western diplomats appear to be softening on their stance that Iran must abandon all uranium enrichment, as is demanded in UN Security Council resolutions, and focusing on the "breakout time" it would take Iran to shift from uranium enrichment to building weapons. By maximising the breakout time, the US could maintain a window for launching military strikes on Iran, should Tehran trigger alerts from UN monitors by switching from peaceful atomic work to weapons-related activities.
This will involve restricting the amount and purity of uranium Iran can enrich; the number of spinning centrifuge machines it can keep and use to refine uranium; the type of research carried out; and the ways in which UN inspectors keep tabs on nuclear sites.
"There are hardliners in Iran and the US who oppose the nuclear agreement. In both countries they are for the moment reined back by the country's leader."
Other hurdles include the Fordow underground enrichment site and a planned reactor at Arak. Iran says it will produce medical isotopes, but Western diplomats see it as a potential site for making weapons-grade plutonium.
Obama estimates the chance of a deal at no more than 50 percent. While the prospect of improved US-Iranian relations and forestalling more conflict in the Middle East is tantalising, many analysts say the US president is too optimistic. "If there is a genuine desire on the part of the Iranians to craft a new relationship with the United States, is that real opportunity going to survive what is a pretty tough political climate between the two countries?" asked Jim Lindsay, from the New York-based Council on Foreign Relations.
As the discussions inch forward, businesspeople have started eyeballing Iran's 80 million consumers, some of the world's biggest oil and gas reserves and an economy worth around $500bn, which would be re-opened by the lifting of UN sanctions.
A desire for trade could pressure some P5+1 members to push for an early lifting of sanctions, particularly if Iran appears to be "reasonable and moderate, and it is the US that is demanding too much," said Muhammad Sahimi, a nuclear expert at the University of Southern California.
But, according to Sahimi, the biggest pressure remains on Rouhani. Iran's relatively moderate president, who replaced the firebrand Mahmoud Ahmadinejad in August, only has a limited time to show that bargaining with the US will work. "Khamenei will not support the negotiations indefinitely. They must bear fruit for Iran and preserve the country's nuclear rights. That fruit must be lifting of the sanctions," he said. "If the US drags its feet and wants Iran to surrender, it will not work."
While Rouhani has to satisfy his conservatives, the same is true in Washington.
"There are hardliners in Iran and the US who oppose the nuclear agreement. In both countries they are for the moment reined back by the country's leader," said Henry Precht, a former US State Department official. "The difference is that in Iran, hard-line mullahs are dependent on their leader. In the US, the president may think he is dependent on the opposition and eventually be obliged to yield."
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