The Syrian regime and its Shia allies are gearing up for a massive offensive in the strategically important Qalamoun region, between Homs and Damascus.
The mountainous region is currently held by some of the most hardline Islamist rebel battalions, including the Al Qaeda-linked Al Nusra Front and the independent Salafist Ahrar al-Sham brigade. It is the corridor to the Sunni Lebanese border town of Arsal, and provides a lifeline to rebel forces, who use it to shuttle men and supplies into the battlefield, and send their wounded to recuperate in the Lebanese town.
Sunni rebels have also reportedly been using the area to set up munitions factories for small arms and mortars.
Arsal has seen its population double to 80,000, as refugees - including families of rebel fighters - have relocated there since the start of the uprising against Syrian President Bashar al-Assad.
The Assad regime still maintains a number of heavily fortified military outposts in the area, which have held out against rebel attempts to take their positions, but the region is mostly controlled by rebels.
A report by the Christian Science Monitor cited sources from both sides of the conflict as confirming that they believed such an operation was imminent, with rebel forces promising to fight to the bitter end to prevent the regime from effectively sealing off the western border.
“We are getting ready to be attacked in Qalamoun. All the factions have set aside their differences and are prepared for the attack. We know it’s coming soon,” CSM quoted a rebel fighter of Lebanese origin, named Khaled, as saying.
Syrian government forces, together with Hezbollah and other Shia militias from Iraq and Lebanon, are hoping to wrest control of the region from rebels in an attempt to restore contiguity between the capital Damascus, through Homs, to the port city of Tartus and the coastal mountains which are the home turf of Assad's Alawite sect.
Rebels are reportedly digging in for a battle royal that is likely to resemble the bloody 17-day fight over the Al Qusair region, also near the Lebanese border. In that campaign, regime forces, aided by Hezbollah, managed to seize control of the strategically-important area from rebels, although they sustained heavy losses in the process.
But rebels - who have reportedly amassed as many as 40,000 fighters in Qalamoun - are promising that this time will be different.
“Qalamoun is not Qusayr,” insists Abu Omar, a resident of Arsal. “The mountains are rugged and we are reinforcing and rearming. We know this battle is coming and we will fight to the last man.”
There are also indications that any Hezbollah involvement in the potentially game-changing battle for Qalamoun could trigger a backlash within Lebanon itself far greater than anything seen thusfar. In that sense, the battle for Qalamoun could be a turning point regionally, as well as locally.
In Lebanon, where some politicians have been talking quite openly about the upcoming Qalamoun offensive, even allies of the Syrian regime are urging Hezbollah not to get involved, fearing the consequences for their own country.
“Saudi Arabia is running the battle in Qalamoun and we have information that it warned Hezbollah against participating in the battle, [or else] it will cost [Hezbollah] a lot in the Bekaa and even in north Lebanon,” said Rifaat Eid, head of the Alawite "Arab Democratic Party," at a recent press conference.
The Bekaa Valley is a Hezbollah stronghold, and has been the site of a number of attacks targeting the group in recent months.
Saudi Arabia is one of the key backers of the Syrian rebels, with the Sunni power using the conflict as a proxy war to fight the growing regional influence of Shia Iran, which backs Assad. Many of the more Salafist-orientated rebel battalions receive their funding from Riyadh, and the Sunni state exerts some influence among Islamist groups within Lebanon as well.
Having poured so much money and resources into the Syrian conflict, the Saudis are unlikely to take defeat in Qalamoun on the chin, and may seek to take the battle to Hezbollah's home territory in response.
For many Sunni Lebanese, Hezbollah's very participation in the bloody and humiliating defeat of the rebels in Qusair, who had been preparing for months alongside Lebanese volunteers, was the straw that broke the camel's back. It sparked a string of apparent revenge attacks by Sunni Islamists against Hezbollah strongholds, including rocket attacks and two car bombs.
The fear is that another Qusair could spell the start of a much wider regional conflagration, pushing tensions between Lebanon's various communities beyond breaking point, and into a civil war of their own.