Here in the Bekaa Valley, we meet Mohammed, Ali and Ghassan – all terrorists, say the US and Australia, by dint of their membership of Lebanon's powerful political party and militia, Hezbollah.

But in taking the fight to the so-called Islamic State and the al-Qaeda-affiliated Nusra Front in the conflict in neighbouring Syria, the three men sing loudly from the Washington and Canberra song sheet – they too are determined to "degrade and destroy" the Sunni extremists now marauding through Syria and Iraq.

We arrive a week too late to meet Ali's 43-year-old brother, Ahmad Saleh, who died along with seven other Hezbollah fighters when their position on Umm Khorj, a forlorn ridge in the Qalamoun Mountains behind Brital (population 20,000), was overrun by Nusra Front and IS fighters on October 5.

"He died on our land defending our village," Ali says. "We are very proud of him – he did it up there so that we'd be safe down here."

A nephew of the dead man looks on as they recount what they parse as Ahmad's heroic end – his unit had tried to hold off the attack by "hundreds" of infiltrators armed with mortars and rocket-propelled grenades until reinforcements had arrived from the town.

But the nephew can only nod his hooded head in agreement or gesticulate in disagreement, because he too was up on the ridge and he took a bullet in the neck – two button-sized wounds, about 10 centimetres apart, mark its entry and exit below his chin. His silence attests to damage inflicted on his voicebox.

Rising to more than 2800 metres, these barren mountains – the Anti-Lebanon range – are a buffer between Lebanon and Syria. And the fighting in and over the mountains confirms the difficulty for all sides in this expanding war in explaining why they are on the battlefield.

The US is calling for the overthrow of the Syrian dictator Bashar al-Assad. But for the most part, the US-led coalition confines its air strikes to targeting the extremist Sunni forces of IS and Nusra, both of which are trying to oust Assad at the same time as they seek to see off the "vetted, moderate" rebel militias sponsored by the West.

Hezbollah, a movement defined by Shiite faith, entered the war in defence of Assad, but now finds that its list of enemies overlaps with that of the Obama coalition. And Hezbollah's pursuit of IS and Nusra is more single-mindedly in line with Washington's objective, because the US-approved rebels reserve the right to fight in concert with Nusra and, sometimes even IS, in a desperate hope that their combined gunfire will take down Assad – after which, they say, they will see off the extremists.

Hezbollah has to be viewed through three prisms.

First, drawing inspiration from Iran's 1979 Islamic revolution, it emerged as a guerrilla group led by Shiite clerics during the Lebanese civil war of 1975-90 and in time, with the patronage of Tehran and Damascus, became a powerful force against Israel – fighting several wars, staring down demands that it disarm and becoming a powerful force in Lebanon's brittle political, security, religious and social dynamics.

Second, Tehran's funding, arming and training makes Hezbollah a powerful arm of Iranian foreign policy in the region, with close links to the Iranian Revolutionary Guard Corps. Assad's defeat would isolate Hezbollah, making it difficult if not impossible to receive weapons and other supplies from Tehran.

Third, this war, in which Hezbollah reportedly has lost as many as 500 fighters, initially left many in Lebanon questioning whether the movement had become more an Iranian weapon than it was a Lebanese resistance movement.

But analysts say that its combat success in pushing IS and Nusra back into Syria, and away from the Lebanese border, while aiding Assad has, for now at least, also spared Lebanon from a full-blown outbreak of the brutal chaos now engulfing much of Syria and Iraq.

Not that Lebanon has been immune. Its Shiite and Christian communities are choice targets for IS and its sympathisers in Lebanon's Sunni community, some of whom have launched revenge attacks on Hezbollah and Shiite communities as punishment for their support for Assad – Hezbollah was instrumental in staving off defeat for the Syrian regime by restoring control of key centres and critical supply routes.

Towns and villages up and down the Bekaa Valley and Hezbollah buildings and the Iranian Embassy in Beirut have been targeted with suicide bombings, rocket fire or ground force assaults.

There have also been tit-for-tat kidnappings and abductions, most recently by IS and Nusra of more than 30 Lebanese police and soldiers from Arsal, in the Bekaa Valley. At the weekend, their IS and Nusra captors allowed a 48-hour deadline they had set for the beheading of the hostages unless Beirut agreed to a prisoner swap to pass without public comment.

Dozens of lampposts on the road into Hermel, at the northern end of the Bekaa Valley, serve as memorials for Hezbollah's war dead – two poster-size images of the dead hang from each post. Those from which the colour has faded died in clashes with Israel; those with full colour are from the war in Syria – and the task has become so big that a contractor has been retained to prepare and hang the posters. "He's making up 12 more at present," I was told.

In the shade of olive and citrus trees in his walled garden on a hill above the town, 45-year-old Ali Awad, secretary of the local branch of the secular Syrian Social Nationalist Party, which operates on both sides of the border, speaks of three car bomb attacks which killed 11 people in the town, adding: "People are afraid – we don't know what will happen tomorrow."

It would not need "much of a push" for IS to take root in the Lebanese Sunni community because events were conspiring to drive the Sunnis to extremes, Awad said. Then there would be a surge of counter-movements – the consequences of which, he said, would be the destruction of the country.

The threat on which most in the Bekaa Valley focus is up in the mountains – thousands of IS and Nusra fighters, local sources insist, who had retreated to the high country when Hezbollah dislodged them from towns and villages on the Syrian side of the mountains.

"We've locked them in, but they keep trying to break out," a Hezbollah figure said, alluding to the attack in which Ahmad Saleh died. "The difficult terrain would make rooting them out a costly battle for us to go up and take them on. But we have them surrounded and we'll wait for the snow and cold of winter to drive them out.

"They only have caves for shelter, so they'll have to move before winter – to just sit up there would mean certain death."

Hezbollah, rated by analysts as better armed and trained than Lebanon's national army, has forged a new bond with Lebanon's national security service – sharing intelligence in a swap arrangement in which Hezbollah is said to have won access to US intelligence.

There are signs that Hezbollah perceives a possibility of Washington becoming less implacable about its blacklisting of the movement – a hope that the Iraqi intelligence services can serve as covert intermediaries with Washington, in the same way as they do for Damascus and Tehran. In a recent interview with The New York Times, the Beirut-based analyst Kamel Wazne argued that if Washington had declared the Sunni jihadists of IS to be the greatest threat in Syria, then it needed to rethink its attitudes to Hezbollah.

"Hezbollah is not representing an imminent threat against the world," he said. "It represents a threat against Israel, as Israel represents a threat against Lebanon. But Hezbollah is not going to threaten the US and Europe. Nobody said Hezbollah is cutting off heads."

In a paper for the Washington Institute for Near East Policy, analyst Mona Alami argues that Hezbollah successfully turned around initial Lebanese disquiet over its entry into the Syrian conflict by blitzing the media with tales of its battle success; cooperating with the Lebanese military; becoming more pragmatic in Lebanese politics and, by the by, becoming more powerful along the way.

Finally, Hezbollah had cracked down on dissent in the Shiite community, she says. Families of the war dead had been banned from media interviews and a newspaper editor told Alami that vandalism of his home was part of "a vast intimidation campaign that is invisible because people do not report it".

That ban on the families of "martyrs" talking to the media seemingly counts for little in Brital, which locals acknowledge in media reports as the centre of a thriving car theft ring that – somewhat embarrassingly for a Shiite community – is believed to supply some of the vehicles used as car bombs against Hezbollah and Shiite targets.

But Ali, the brother of dead Hezbollah fighter Ahmad Saleh, digs into history for more satisfying boasts – neither Lebanon's Ottoman nor French occupiers had been able to enter Brital; the Syrians too had failed; and Hezbollah's first "martyr", Abbas Saleh, had been a member of his own extended family.