DUBAI (Reuters) - An interim international deal on Iran's nuclear programme could tilt the balance of power in the Middle East towards Tehran after two years of popular revolts that have weakened leading Arab nations.
Sunday's agreement opens the way for a thaw in U.S.-Iranian confrontation that has lasted almost as long as the U.S.-Soviet Cold War, alarming Israel and Gulf Arab rulers who fear a new regional hegemon deeply hostile to their interests.
The deal to curb but not scrap Iranian uranium enrichment, which the West has long believed was meant to develop a bomb, has implications far beyond weapons proliferation in a war-scarred region critical to world oil supplies.
For some Gulf Arab states, which see Tehran as a regional troublemaker, and for Israel, which regards Iran as a mortal threat, the Geneva agreement means they have failed to dissuade Washington from a course they suspect will end in tears, such is their distrust of the Islamic Republic.
Iran will grow richer and stronger through the easing and eventual lifting of sanctions that have shackled its economy, emboldening its Islamist rulers to step up support to Shi'ite Muslim allies in Arab countries, critics of the deal say.
In contrast, supporters of the accord say a rapprochement between two powers so long at odds could help stabilise a region in turmoil and reduce sectarian strains that have set Shi'ite and Sunni Muslims bloodily against each other.
Mistrust has been mutual, as it was in the post-World War Two impasse between the West and the Soviet Union.
The United States and Iran have had no official ties since 1980 after Iranian students occupied the U.S. embassy in Tehran, taking 52 diplomats hostage in protest against Washington's admission of the former Shah after he was toppled by the Islamic revolution.
With the historic Arab power centres of Egypt, Syria and Iraq all weakened by uprisings and sectarian strife, a new start with Tehran has emerged as an enticing potential win for a U.S. administration in search of a foreign policy success.
Rami Khoury of the American University of Beirut described the interim deal restricting Iran's nuclear work as a "very good thing" that could eventually lead to rapprochement between Tehran's clerical rulers and U.S.-allied Gulf Arab states.
"If the negotiations continue on and keep working, and the sanctions are slowly removed, it will revive Iran's economy, and eventually its liberal movement and I think we will slowly see social and political progress in the country," Khoury said.
"In the short run it encourages cooperation between the United States and Iran to try and deal with Syria and stop the violence there. Right now there is a common threat developing, the (Sunni militants) who will bomb the Iranians, bomb the U.S., as we've seen, so they're everybody's enemy right now."
Experts say Gulf Arab countries will try to piece together a diplomatic and security strategy with like-minded countries to reduce their vulnerability to a resurgent Iran now eagerly contemplating a future free of crippling sanctions.
For his part, Israeli Prime Minister Benjamin Netanyahu said the interim deal was a historic mistake because "the most dangerous regime in the world took a significant step towards obtaining the world's most dangerous weapon."
He reaffirmed a long-standing Israeli threat of possible military action against Iran, although a member of his security cabinet acknowledged the interim accord limited that option.
At the heart of Gulf Arab concerns is a belief that the moderate Iranian officials who negotiated the nuclear deal are not the hard men in charge of what they see as Shi'ite meddling in Sunni Arab countries. Those forces remain dominant in the Revolutionary Guards and intelligence services.
Gulf Arabs cite as a prime example Iranian support for Syrian President Bashar al-Assad, member of a sect that is an offshoot of Shi'ite Islam, who has waged a 2-1/2-year-old war against mostly Sunni rebels backed in part by Gulf Arabs.
A senior Gulf Arab official close to Saudi government thinking told Reuters the kingdom's attitude continued to be characterised largely by "suspicion", based on Iranian involvement in Syria, Yemen and Bahrain.
"The atomic arsenal is not their only arsenal - it is the mischief arsenal they have that worries us," he said.
"We have had a lot of accords and promises from them in the past. Now, we hope to see a corrective process with this deal."
The disclosure that senior U.S. officials held secret bilateral talks with Iranian counterparts in recent months to prepare for the nuclear agreement may exacerbate Gulf Arab rulers' fears that Washington is willing to go behind their backs to do a deal with Iran.
Many Gulf Arabs suspect that the commercial imperatives that have driven decades of U.S. engagement with them are similar to those driving U.S. outreach to Tehran - business.
"The U.S. has its interests - Iran is a lucrative market. Iranians need a lot of infrastructure for rebuilding that could generate billions of dollars for U.S. and U.K. oil companies," said Abdullatif al-Mulhim, a retired Saudi navy commodore and now a newspaper commentator.
In addition, some Gulf Arabs fret that a United States increasingly self reliant in energy thanks to domestic shale gas might be less committed to guarding the Strait of Hormuz, the narrow artery through which 40 percent of global sea-borne oil exports pass.
Sami al-Faraj, a security adviser to the Gulf Cooperation Council (GCC), said Gulf Arab governments would now work diplomatically and on the security level to ensure they were adequately protected against any resurgent Iranian ambitions.
Gulf Arabs felt slighted by the deal, he suggested.
"Iran is sitting at the high table. We are left with the leftovers."
He added: "We will acquire more weapons...We are going to check if our shopping list is adequate to respond to this."
"We are going to rally other nations that are hurt by this action into a unified diplomatic campaign," Fajr said.
Warming U.S.-Iranian relations could help Assad in Syria.
Some analysts speculate that Washington's need to protect what could become one of its few diplomatic achievements in the region will mean that it will do whatever it needs to keep the Iranian thaw on track.
"Now, the U.S. is even less likely to put serious pressure on Iran over their support of the Assad regime during the negotiations," said Shadi Hamid of the Brookings Doha think-tank.
"And, obviously, with everyone's attention on Iran, Assad has cover to do pretty much whatever he wants."
Emile Hokayem of the International Institute of Strategic Studies, played down the idea that Israel and Gulf Arab states would make common cause in any systematic way against Iran.
"It's a convergence of interest, it's not an alliance," he said. "Each of them could reinforce messages on Capitol Hill, but don't be too carried away by the possibility of direct cooperation."
Additional reporting By Erika Solomon in Beirut and Rania El Gamal in Dubai, Dan Williams and Maayan Lubel in Jerusalem; Editing by Paul Taylor