In the photograph the two robed men stand shoulder-to-shoulder, one tall and erect, the other more heavyset. Both smile for the camera. The picture from Tehran is a rare record of Iran's Supreme Leader Ayatollah Ali Khamenei meeting Sayyed Hassan Nasrallah, the head of Hezbollah, the Lebanese Shi'ite paramilitary group.
Taken in April during a discreet visit by the Hezbollah chief to his financial and ideological masters, the photograph captured a turning point in Syria's civil war and the broader struggle between Sunnis and Shi'ites, the two main branches of Islam. It was the moment when Iran made public its desire for Hezbollah to join the battle to help save Syria's President Bashar Assad, diplomats said. At the time, Assad and his Alawite sect, an offshoot of Shi'ite Islam, were losing ground to an advancing Sunni insurgency.
Within days of returning home, Nasrallah gave a televised speech making it clear that Hezbollah would fight alongside Assad to prevent Syria falling "into the hands" of Sunni jihadi radicals, the United States and Israel. The very survival of the Shi'ites was at stake, he said.
Soon afterwards, fighters from Hezbollah - which until then had largely stayed out of its neighbor's civil war - entered Syria. In June they helped Assad's forces recapture the strategic town of Qusair and other territory, turning the war in Assad's favor.
Regional security officials told Reuters there are now between 2,000 and 4,000 Hezbollah fighters, experts and reservists in Syria. One Lebanese security official said a central command in Iran led by the Revolutionary Guards directs Hezbollah operations in Syria in close coordination with the Syrian authorities. Another source said Hezbollah had "hit squads" of highly trained fighters in Syria whose task is to assassinate military leaders among the Sunni rebels.
Hezbollah declined to comment for this report on its involvement in Syria. Nasrallah has previously said it is necessary for Hezbollah to fight Sunni radicals allied to al-Qaida.
Officials in Iran did not respond to requests for comment. Last week, Iran's foreign ministry spokeswoman, Marzieh Afkham, said that Iran had no official military presence in Syria, but was providing humanitarian assistance. Last September, Mohammad Ali Jafari, head of the Revolutionary Guards, said some members of Iran's elite Quds force were in Syria but that it did not constitute "a military presence."
Hezbollah's role in Syria has ramifications not just in its home in Lebanon but across the region. If Assad wins, Iran's influence along the shores of the Mediterranean will grow. If he loses, Hezbollah and Iran's reach will likely be damaged. For some members of the group, the fight is an existential one.
Reuters has learned that a few voices within Hezbollah, which is considered a terrorist organization by the United States and Europe, opposed joining the conflict in Syria. Two prominent members feared intervention would drag Hezbollah and the Shi'ite community into a quagmire; they questioned where the group would draw the line after Qusair.
Sheikh Subhi al-Tufayli, who led Hezbollah from 1989 to 1991, said the decision to intervene had been entirely down to the Islamic Republic of Iran. "I was secretary general of the party and I know that the decision is Iranian, and the alternative would have been a confrontation with the Iranians," Tufayli, who fell out with Iran and his former group, told Reuters at his home in the Eastern Bekaa Valley near the Syrian border. "I know that the Lebanese in Hezbollah, and Sayyed Hassan Nasrallah more than anyone, are not convinced about this war."
Such doubts are repeated across the Middle East. Shi'ite groups, clerics and communities in places such as Iraq are struggling with whether to back Assad or not.
But the critical voices were ignored and eventually silenced. "Even if (Hezbollah) has its wise men, the decision (to fight in Syria) is not theirs," said a Lebanese security official who, like most people Reuters spoke to for this report, would not be named. "The decision is for those who created and established it. They are obliged to follow Iran's orders."
A Lebanese politician summed up the point, saying: "Nasrallah is not going to say ‘No' to someone who has given him $30 billion over the past 30 years."
The paramilitary group - its name means the Party of God in Arabic - was originally conceived at the Iranian embassy in Damascus in 1982. Its main aim was to fight Israeli forces that had invaded Lebanon that year.
It became notorious for suicide-bombings, kidnappings and hijackings as it drove Israel back towards its border with Lebanon; it also pushed US and European forces out of Beirut following the Israeli invasion and during Iran's war against Iraq, which the West had armed and backed.
Hezbollah came to serve as a subcontractor buttressing the strategic interests of its Iranian paymasters, forming a military front with Syria and Iran against Israel and the United States. Domestically, it spearheaded the rise of Lebanese Shi'ites from an underclass community to, by some lights, the most powerful sect in the country.
Its paramilitary forces are now more powerful than the Lebanese army and even some Arab armies, regional experts say. It has an Iranian-trained strike force numbering around 7,000, with some 20,000 reservists, according to security officials and diplomats.
In Syria, the discipline and training of Hezbollah fighters paid off most significantly in June, when Assad's regime recaptured the town of Qusair, about 10 km (6 miles) from the Lebanese border. A regional security official said: "(The battle for) Qusair was basically a Hezbollah operation, from the planning to the handling of key weapon systems. It is our understanding the Hezbollah crews were even operating Syrian T-55 and T-54 tanks there, as well as all significant artillery systems, anti-tank missiles and so on."
Since then, Hezbollah has expanded its deployment in Syria to every area where rebels are present, a regional security source who declined to be identified said.
The group has beefed up its presence around the capital Damascus, the border area and the city of Homs, which is strategically located between Damascus and the mountain heartland of Assad's minority Alawite sect.
Its main task is to prevent rebel groups, mainly Sunni jihadis linked to al-Qaida, such as the al-Nusra Front, from entering the heart of the capital. "It is (Shi'ite) Hezbollah versus (Sunni) al-Nusra Front and other jihadis now in Syria," said one military observer.
The regional security source said: "In these places, Hezbollah is hunkering down in fixed positions because it understands that the fighting will be protracted and will shape its fate in Lebanon. Its actions are taken in full coordination with the Syrian military, and Iranian experts provide it with military and technological assistance."
Hezbollah is also putting down roots in Bosra al-Sham, south of Damascus, and other places on the Syrian side of the Golan Heights, a strategic plateau in southwestern Syria that is held by Israel, said the source. The group wants to prevent weapons being sent from Lebanon to rebels in Syria, and to stop rebels moving the other way. To do so, it sets up ambush points and lays mines on cross-border routes, said the regional security source.
"Where in the past Hezbollah deployment in Syria was focused on protecting Shi'ite populations, now it is everywhere there is fighting with the rebels," said the source.
Hezbollah fighters serve as the prime instructors for the Syrian militias that provide Assad's most loyal forces, said the same source. "Hezbollah also has hit squads, covert units selected from among its best fighters and trained by Iranians, whose mission is to assassinate Sunni opposition leaders and Free Syrian Army commanders in Damascus and Aleppo," he said.
Hezbollah did not comment on its involvement in Syria.
As well as its standard weapons, Hezbollah is using new arms, mostly from Iran, that are flown in to Damascus or Beirut. Hezbollah has also received weapons from the Syrian army, including flame throwers, said the source.
Weapons are moved from Lebanon into Syria with high frequency and little difficulty, given the control that the Syrian regime and Hezbollah wield over the border crossings, the source said.
The Iranian Revolutionary Guards Corps and Quds force, and the Syrian military high command, operate a war room to coordinate Syrian army and Hezbollah operations. This war room was initially responsible for deploying Hezbollah fighters in Syria on specific operations. But more recently, "Hezbollah was ... given responsibility over geographical areas as well as over security installations," said the source.
Supporting its fighters in Syria is Hezbollah's network of political and commercial interests in Lebanon. The group now has 12 seats in Lebanon's parliament, two ministers in the current caretaker cabinet, a radio and satellite television station, and a community network that provides everything from health and education to pensions and housing.
As well as penetrating the army and security services, it places allies in every significant ministry, government office, or state-owned enterprise and key institutions, according to Lebanese political and security sources.
At the Beirut harbor, Hezbollah has a dock of its own, according to two Lebanese security sources. Shi'ite merchants linked to Hezbollah bring consignments through the dock to avoid paying custom duties, sell them at prices lower than competitors, and donate some of the profits to the group, the security and political sources said.
In addition, the group has investments in Lebanon and abroad, including construction, supermarkets, petrol stations, and industry projects. "They have their own money-laundering operations," one Lebanese politician said. "They legalize hot money through high cash-generating businesses and front companies such as real estate, cell phone shops, valet parking companies and religious foundations."
Former US Treasury official Matthew Levitt, a fellow at the Washington Institute and author of the forthcoming "Hezbollah: The Global Footprint of Lebanon's Party of God," said the group is engaged in a broad array of illicit activities, from counterfeiting currencies, documents and goods to credit card fraud, money-laundering, arms smuggling and narcotics trafficking. Hezbollah, one investigator quipped, is like the "Gambinos on steroids."
Hezbollah has regularly denied such allegations.
Politically, Hezbollah can make or break Tammam Salam, the Sunni politician tasked in April with forming a new government in Lebanon. The group enjoys a veto on all policy decisions - a power it secured after a long standoff between it and the Sunni-led government which began after the 2006 war with Israel.
"If Hezbollah wants to form a government then it will be formed; if they don't, it won't. They are the most powerful force on the ground. They are more powerful than the state," said a Western diplomat.
Hezbollah's creeping hegemony in Lebanon began after the 2005 killing of former prime minister Rafik al-Hariri, a Sunni Muslim billionaire who used money, influence and international clout to win support across Lebanon's sectarian divides.
Hariri, who had close links to Saudi Arabia and the West, was assassinated in Beirut by a car bomb in which UN investigators saw the trademark handiwork of Syria, and for which four Hezbollah members were subsequently indicted. None of the four has been arrested. The group denies any involvement in the killing.
Hariri's killing prompted an international outcry which forced Hezbollah's ally Syria to end its 29-year military presence in Lebanon. But it also removed from the scene the one man who could have challenged Hezbollah's dominance.
"If he were still alive, he would have had the majority in government and the position of (Hezbollah) would have been difficult," said Tufayli, the former Hezbollah leader.
The Lebanese security figure said Hariri was killed in a joint Iranian-Syrian plan executed by Hezbollah elements without Nasrallah's knowledge.
Hezbollah has shown itself unwilling to countenance the smallest threat. In June when unarmed Shi'ites protested outside the Iranian embassy in Beirut against Iran's military involvement in Syria, Hezbollah gunmen, dressed in black and armed with handguns, charged the crowd, killing one protester.
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Syria presents wider risks. Under Nasrallah, Hezbollah initially tried to maintain a balance between its role in Lebanon and its ambitions as an Islamist vanguard of Iran in the region. The intervention in Syria has ended this ambiguity, placing Hezbollah in the frontline of the regional conflict between the Western-backed Sunni Arab powers and Shi'ite Iran.
The chaos threatens to unleash sectarian demons from Beirut to Baghdad. Reprisals against Hezbollah have already begun: In May, rockets were fired at the Hezbollah-controlled southern suburbs of Beirut, and since then several car bombs have exploded in Lebanon.
"Hezbollah entered a Sunni-Shi'ite conflict declaring jihad, so they should expect counter-jihad in return," said one Sunni opposition figure.
Tufayli, the former Hezbollah leader, said the group's intervention in Syria was a fatal miscalculation. The conflict, he said, is becoming a sectarian proxy war that minority Shi'ites will never win.
"Until recently, I had thought that armed resistance (against Israel) is a top priority and a precious goal... Those seeking to fortify the resistance should not drag it into war between Sunnis and Shi'ites... That strife will consume everybody," he said.
The war is imposing huge costs on both Hezbollah and Iran, which is already under crippling international sanctions because of its nuclear ambitions.
A regional security official with access to current intelligence assessments put Hezbollah's annual income at between $800 million and $1 billion, with 70-90 percent coming from Iran, the amount partly depending on the price of oil. The group's remaining funds come through private Shi'ite donors, "protection rackets and business and mafia networks in Lebanon," said the source.
Apart from its involvement in Syria, Hezbollah pays salaries to 60,000-80,000 people working for charities, schools, clinics and other institutions in addition to its military and security apparatus, other Shi'ite sources said.
Other security sources said Hezbollah is now receiving additional funds dedicated to the Syrian war. "Syria is sucking up Iran's reserves, with the Islamic Republic paying between $600-700 million a month (just towards the cost of fighting in Syria)," said a top Lebanese security official. Those figures could not be confirmed.
And the price is not just financial: Hezbollah's involvement in Syria has hurt its support at home. "There isn't a single village in the south that has not lost a member (in Syria)," said Ali al-Amin, a Shi'ite columnist and a critic of Hezbollah.
Most Lebanese Shi'ites, though, still support the group. "A large chunk of society is rallying behind Hezbollah because they regard their ties to it as existential," said Amin. "They say 'we are with it whether it goes to hell or heaven.'"