Hassan Nasrallah, Hizbollah leader

The stream of coffins, wrapped in the strident yellow banners of Hizbollah as they trickle across the Syrian border to Shia strongholds in Lebanon, tells the story. Mr Hizbollah, Hassan Nasrallah, has been sucked into the vortex of Syria’s two-year-old civil war: committing the full might of his Shia Islamist militia to help Bashar al-Assad finally crush an insurgency dominated by the country’s Sunni majority. It is a fateful decision, which could see the legend of Hizbollah expire, along with its leader’s, in the rubble of al-Qusair.

This is the small town astride the strategic Homs Gap linking the capital, Damascus, to the northwest coastal heartland of the minority Alawites, the esoteric branch of Shiism the Assads made the backbone of their crumbling security state. Qusair is also a staging post for arms smuggled from Lebanon to the rebels. Assad forces have failed several times to clear this corridor.

Now, in their new role as shock-troops for a regime that is morphing from a state into a well-armed militia network, Hizbollah, supported by pitiless air strikes and shelling, has taken on the task. Though it is hard to see how the hard-pressed rebels can avoid eventually being outflanked, Mr Nasrallah’s finest have taken a pasting.

No matter how this battle ends, everything has changed for Sayyed Hassan Nasrallah. Widely celebrated across the Muslim world as the Arab champion who stood up to Israel, and among his own as the man who raised up the Shia underclass of Lebanon, he is now being painted as the praetorian of Syrian tyranny and a stooge of Iranian theocracy.

Mr Nasrallah’s soaring oratory is buckling under the strain of portraying Hizbollah’s nakedly sectarian support for the Assads – described by Syrian rebel leaders this week as an invasion – as a divinely sanctioned act of resistance. This is quite a turnround for a man and a movement forged by another invasion, by Israel of Lebanon amid the turmoil of Lebanon’s sectarian civil war of 1975-90.

The Party of God, set up in Iran’s embassy in Damascus in 1982, became notorious for suicide bombings and kidnappings as it drove Israel towards its borders and US and European forces from Beirut. Mr Nasrallah took over Hizbollah after his predecessor, Abbas Mussawi, was assassinated by Israel in 1992. He made it a formidable guerrilla army and a state-within-the-state able to dominate Lebanese politics. A mere 32 at the time, he was a rare combination: shrewd and cerebral, fiery and charismatic. He extended the reach of Hizbollah’s mini-state in south Lebanon and the Dahiyeh, the southern suburbs of Beirut; and from Iran-financed welfare into education and health, housing and microfinance. His gift for propaganda, first evident through al-Manar, the movement’s television station, has kept up with new media. Hizbollah politicians cultivate an image of probity and efficiency.

In an interview with the FT in 1998, Mr Nasrallah argued that Iran under the reformist leadership of then President Mohammad Khatami would light the path of Islam and eclipse obscurantists such as Osama bin Laden, whose followers had just bombed US embassies in east Africa.

Yet Hizbollah, licensed by Syria during its occupation of Lebanon as a resistance movement to Israel’s occupation of the south, always thrived on conflict. After forcing Israel out of south Lebanon in 2000, Mr Nasrallah’s apotheosis came in the war of 2006, in which Hizbollah held Israel to a bloody draw. Many Lebanese contrasted the “divine victory” Hizbollah proclaimed with the devastation Israel visited on the country after the group kidnapped three of its soldiers, prompting Mr Nasrallah into a partial mea culpa. The turban had started to slip.

Hassan Nasrallah was born into a poor Shia family in 1960 in Karantina, a portside slum in Christian east Beirut. He studied theology in Najaf, the Iraqi Shia shrine city, until expelled by Saddam Hussein in 1978. A teacher by inclination, he fought in the Lebanon war. As the war drew to a close he resumed his studies at Qom in Iran, becoming Hizbollah’s man in Tehran before taking over the leadership.

Seasoned Hizbollah watchers believe from then on he became a soldier of Ali Khamenei, Iran’s supreme leader, who ultimately placed his guns at the service of the Iranian Revolutionary Guards. Certainly, he avoided choosing between being a legitimate actor in Lebanon or the spearhead of Iran in the Levant. That choice has now been made, possibly for him by Iran.

Early on in the Syrian conflict, Mr Nasrallah – urged by Khaled Meshaal, the Hamas leader then based in Damascus – tried to talk Mr Assad out of war and into reform. He failed. But whereas Mr Meshaal abandoned Syria, Mr Nasrallah has embraced the regime as a bulwark against Sunni jihadi fanaticism. That increases the risk the conflict will spill over into regional war. Israel has already bombed what it says were Iranian rockets transiting Syria for Hizbollah three times this year. The group’s protagonism now makes such attacks, and rebel reprisals inside Lebanon, more likely. The Dahiyeh was rocketed last Sunday.

After 21 years of Mr Nasrallah, Hizbollah has grown fat and prone to scandal, from involvement in money laundering to being accused of the 2005 assassination of Rafiq Hariri, Lebanon’s former prime minister. It started looking like an Iranian proxy when it over-ran west Beirut after a long stand-off with domestic, mainly Sunni opponents in 2008.

While Hizbollah is straining to keep the peace inside Lebanon, partly by increasing its penetration of the army and security services, its deepening involvement in Syria will limit its ability to keep the lid on at home – unless it takes greater control of the country. Mr Nasrallah says he believes no faction inside Lebanon can impose its will on the country’s 18 sects. But now that he is risking Hizbollah’s survival in the blood-spattered arena next door, he risks being caught on the wrong side of history, there and at home.

The writer is the FT’s international affairs editor