On Saturday, after 13 years of tension and negotiations, Iran and the international community celebrated "Implementation Day" of a deal over Tehran's nuclear program.
The Islamic Republic has given up any potential for a nuclear weapon. It has shipped almost all enriched uranium outside Iran, reduced uranium centrifuges by 70%, and re-designed a heavy-water nuclear reactor to prevent plutonium by-product which could be used for a bomb. It has acceded to the Additional Protocol of the Nuclear Non-Proliferation Treaty, with more extensive supervision and inspection of Iranian facilities.
In return, the US and European Union will begin lifting sanctions which have crippled the Iranian economy and cut its oil exports by 40% since 2012. An estimated $100 billion of Iran's assets will be unfrozen. Foreign investors will be able to pursue opportunities in a country of 80 million people, including in Iran's oil and gas sector.
But will the deal hold after the initial enthusiasm? The answer to that question will not come from Washington, where - despite inevitable sniping from Republican Presidential candidates, some hostile legislators, and conservative think tanks - the Obama Administration has embedded the agreement as a political reality.
Instead, any challenge will come from Tehran. The Iranian regime is beset by in-fighting where one of its leading factions might see an advantage in turning on the deal. The Revolutionary Guards, already opposed to the Rouhani Government over its handling of foreign policy, could see their extensive interests in Iran's economy threatened by the influx of foreign investors and traders. The Supreme Leader, who has reluctantly accepted the nuclear negotiations, is bitterly hostile to further discussions with the US.
Embedding the Deal in Washington
All the Republican candidates in last week's Presidential debate derided the Obama approach to Iran, with some even thinking of a call for war. Israeli Prime Minister Benjamin Netanyahu signaled to American supporters with the ritual declaration that Iran has not given up its ambition for a nuclear weapon. Some members of Congress vented their frustration in statements and Twitter messages.
But the cold political reality for those critics is that they finally lost the contest last summer. The Obama Administration conceded a review period in which Congress could vote against the July 2015 agreement, but opponents could not muster a majority, let alone the 2/3rds needed to override a Presidential veto.
Having pursued the deal for years, the Administration had changed the political "burden of proof". For years, critics had maintained the threat of an imminent Iranian nuclear weapon - however dubious the claim - over the benefit of an agreement to ease that prospect. Now the President, the State Department, and US military had established the positive outcome of a relationship based on discussion and diplomacy over that based on the possibility of war.
That shift was confirmed last week when Iran's detention of 10 US Navy personnel, following an incursion by two boats into Iranian waters, could have unsettled Implementation Day. US Secretary of State John Kerry and Iranian Foreign Mohammad Javad Zarif urgently conferred in a series of phone calls, and the Revolutionary Guards released the sailors in less than 24 hours. Both Kerry and Zarif proclaimed that calm diplomacy had triumphed.
Instead of turning into crisis, the episode was overtaken on Saturday by the surprise announcement - after 15 months of secret negotiations - that five US detainees in Iran would be swapped for seven Iranians convicted or indicted in America.
The Threat from Tehran
Iran's regime is beset by some of its most serious in-fighting since the 1979 Islamic Revolution. Hardliners and some conservatives were unsettled by the surprise election of "centrist" President Hassan Rouhani in 2013. Now they fear that a centrist bloc - allied with reformists who have been suppressed within Iran for more than a decade - could gain influence in February's elections for Parliament and the Assembly of Experts, the body which chooses the Supreme Leader.
The nuclear deal was insulated from this in-fighting only because the Supreme Leader reluctantly supported the negotiations, following Rouhani's explanation that the alternative was the collapse of the Iranian economy.
The President may still hold that economic trump card, with his promise of 5% growth this year. However, he has other problems. Iran's involvement in regional crises such as Syria's civil war and the breakdown of relations with Saudi Arabia have damaged Rouhani's foreign policy of "engagement". Instead, the Revolutionary Guards and other hardliners are challenging a "weak" Government and saying that a tough line must be maintained with "enemies" such as the US, the Saudis, and "Zionists".
For now, the Supreme Leader is holding the balance by supporting the nuclear agreement while denouncing Washington and Riyadh.
But that balance could easily be lost. If the hardliners are dissatisfied with the outcome of the elections, they could press the Supreme Leader to pursue his "Resistance Economy", favoring autarchy rather than links with the West. If the Guards and their allies believe that Iran must be more aggressive in regional contests, they could unsettle the co-operation of the deal. Ballistic missile testing could be used as a continued taunt to the West, forcing the US to swallow its opposition or step up sanctions - which in turn would feed Iranian critics of the agreement.
A longer version of this article can be found at The Conversation.