Beirutis like to say that their city thrives on uncertainty. "We've been through worse," is a common refrain. "We're used to war every few years," is another.

In the last few months, though, bravado has been replaced by uncertainty and fear. Residents are often heard discussing the steadily deteriorating region in more foreboding tones.

"Is war really coming?" they regularly ask each other. Amid the rumble and whirl of drills and construction cranes, many in Beirut prefer not to draw conclusions. But away from the capital, the countryside resounds to the unmistakable drumbeat of war.

The largely Sunni north has taken on an increasingly heavy burden as Syria has unravelled. Lebanese men have gone to fight on Syrian battlefields, from where hundreds of thousands of civilians have fled to Lebanon.

Two years of sporadic clashes between Sunnis in Lebanon's second city, Tripoli, and a minority Alawite Shia community barricaded on a residential hilltop have recently taken the shape of a more enduring battle.

Here, the Syrian civil war is unmistakably cast as a sectarian bid, led by Iran, to keep Sunnis away from power in the Levant. Fighting has intensified in each of the last three weeks, as Hezbollah – the Shia militia-cum-political powerhouse – has emerged from the shadows to take a very public stake in Syria's war.

The speech two weeks ago by Hezbollah's leader, Hassan Nasrallah, announcing his militia's role in seizing from rebels the border town of Qusair has heightened tensions. There is an undeniable sense that a reckoning now awaits the Shias of Lebanon, and especially their patrons in Tehran and Damascus. Nasrallah's belligerent speech has done far more than the two-year creep of chaos across the Lebanon ranges to crystallise what is now at stake.

Hezbollah's victory in Qusair, on behalf of Assad's regime, is widely viewed as a first step in the escalation of the group's role on other Syrian battlefronts. Many Sunni communities in the north are increasingly viewing the conflict in straight-up sectarian terms, believing they are being inexorably drawn into a fight that extends well beyond Lebanon's borders.

The Shia of the south, meanwhile, cast Hezbollah's role in Syria as a pre-emptive bid to protect them from an ancient inter-Muslim foe, salafists or takfiris – fundamentalist streams of Sunni Islam who the Shia claim are trying to attack them. This mutual demonisation is clearly hardening sectarian positions in the south and north. It is also being felt in parts of the capital, where both sects live alongside each other. Here, tensions run just as high as in the respective heartlands.

In Lebanon's moribund parliament, though, there seems to be some kind of a detente at play. "Hezbollah sends us messages constantly that they don't want things to get out of hand here," said one member of the opposition March 14 political bloc. "We believe them about that. But what has been unleashed could prove unstoppable."

Meanwhile, Beirut's construction boom – legacies of contracts signed in better years – continues unabated. Hotels, however, stand largely empty and high-street shopping strips are deserted. Lebanon is not yet a country at war, but nor is it at peace with itself.