Opposing sides in Syria’s grueling civil war are girding for the opening of a key battlefront in the coming weeks, with expectations that the fighting will spill over into Lebanon.

The Syrian Army, backed by militants from Lebanon's Hezbollah, is poised to launch an offensive to dislodge rebel groups from the Qalamoun region, a strategic area of arid mountains lying between Damascus and Homs, adjacent to Lebanon’s eastern border. The rebels are aware and are preparing to defend their ground, which has been an essential staging ground for attacks.

“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,” says Khaled, a stocky Lebanese Sunni from Lebanon’s northern Bekaa Valley who has fought with Syrian rebel groups since the early stages of the uprising against the regime of Syrian President Bashar al-Assad.

Khaled and other Syrian and Lebanese fighters interviewed say they believe the attack will begin before the onset of the winter rains, which suggests within the next month. Sources close to Hezbollah, which is expected to play a major role in an attack on Qalamoun, say it will begin soon after the three-day Eid al-Adha Muslim holiday, which ended Thursday.

Both the Assad regime and rebel forces have a vested interest in controlling the Qalamoun area. For the regime, it represents a vital lifeline connecting Damascus to Homs and to the Mediterranean port of Tartous, the gateway to the coastal mountains running north to Latakia. That is the heartland of Syria’s Alawite community, a Shiite splinter sect to which the Assad family belongs. Regime dominance of Qalamoun would also effectively seal off the eastern border with Lebanon, severing rebel forces in Syria from Arsal, a Lebanese Sunni-populated border town that is a bedrock of support for the Syrian opposition.

For the rebels, holding Qalamoun complicates the regime's goal of establishing a corridor between the capital and the coast and serves as a useful launchpad for the battles around the northern and eastern edges of Damascus. The handful of stony trails that head west from Qalamoun across the Lebanon-Syria border allow fighters to reach Arsal, where they can rest and recuperate from wounds, or visit their families who have sought refuge in the Lebanese town, doubling its pre-war population of 40,000.

Waiting period over

A regime assault on Qalamoun was expected in early summer after the fall of Qusayr, a rebel-held town five miles north of Lebanon's border that was overrun in a 17-day offensive spearheaded by Hezbollah. Most of the survivors from Qusayr fled to the Qalamoun area. Hezbollah fighters said in early July that they had carried out reconnaissance missions in the Qalamoun area and were drawing up plans to drive out rebel forces.

The offensive was postponed, however, possibly due to the bitter fighting in recent months in the northern and eastern neighborhoods of Damascus, where regime troops backed by Hezbollah and Iraqi Shiite auxiliaries have struggled to break the tenacious hold of rebel forces.

However, in the past three weeks, the long-awaited Qalamoun offensive has seemed increasingly imminent.

Lebanese politicians allied with the Assad regime have discussed the upcoming attack in public, and some have warned against Hezbollah's involvement. On Monday, Rifaat Eid, the head of the Arab Democratic Party, an Alawite political body, warned that Saudi Arabia, a key backer of some Syrian rebel factions, was planning to “burn Lebanon” in retaliation if Hezbollah joined the battle in Qalamoun.

“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 [the Shiite party] a lot in the Bekaa and even in north Lebanon,” he said in a news conference.

Rebel buildup

The rebels have used the summer months to build up their forces in the Qalamoun area, particularly with well-armed, well-trained jihadist groups. The leading rebel forces there are the Al Qaeda-affiliated Jabhat al-Nusra, the hardline Salafist Ahrar al-Sham, Liwa al-Tawhid, and Liwa al-Islam, according to rebel sources. According to a diplomatic report prepared by a European embassy in Beirut, the rebels had around 5,000 fighters in the Qalamoun area a few months ago, including 200 from Jabhat al-Nusra and 300 to 400 from Ahrar al-Sham.

“Now, the rebels have in excess of 40,000 fighters in Qalamoun,” the report says, adding that most are affiliated with hardline Islamist factions.

The rebels even have a weapons and munitions production line with mortar tubes, base plates, and rounds being manufactured in foundries in the town of Yabroud. A diplomatic source in Beirut with access to intelligence information said that the rebels are producing up to 40 mortars a day and have even begun manufacturing their own ammunition for AK-47 rifles.

Furthermore, they successfully stormed Syrian Army ammunition depots in the Qalamoun area and seized large quantities of weapons and ammunition. On Aug. 2, rebel groups overran three Syrian Army ammunition storage sites in the Qalamoun area, which yielded large quantities of advanced anti-tank missiles.

Still, the regime has not been routed from the region. The Syrian Army holds a string of bases across Qalamoun, including large missile facilities with bunkers dug into the sides of mountains at Adra and Nasiriyah. The bases, like regime islands in a rebel sea, allow the Army to shell opposition-held areas and have prevented the rebels from completely severing the Damascus-Homs highway, especially the section between Yabroud and Deir Atiyah.

“The Army is strong there,” says Abu Omar, a resident of Arsal who provides logistical support for Syrian rebel factions. “If we fire one shell at them, they fire 400 back at us.”

Even the population of the area is not entirely supportive of the rebel cause. Towns with Christian populations like Maaloula, Sednaya, and Yabroud tend to be more tolerant of the regime and wary of the increasingly Islamist leanings of the rebel groups.

“Seventy percent of Yabroud is with the regime, even though rebels are in the town. The Christians there don’t interfere with us. Flita is with the regime. That’s why all the villages around Flita are shelled but Flita is left alone,” says Khaled, the Lebanese fighter, whose wife is a Sunni from Yabroud.

No borders in this war

Sources close to both Hezbollah and Syrian rebel factions believe that Arsal’s strong logistical connection to the Qalamoun area means that once the offensive begins, the fighting will spill into Lebanon. The cross-border tracks connecting Arsal to Qalamoun are tenuous lifelines for the Lebanese town which is otherwise surrounded to the north, west, and south by Shiite-populated villages with a strong Hezbollah presence. If Hezbollah is able to cut the tracks between Arsal and the border, the Sunni town will become isolated.

Further inflaming tensions between Arsal and nearby Hezbollah-supporting villages are the allegations that residents of the Sunni town have been responsible for a string of attacks against Shiite areas, including rocket barrages and roadside bombs.

Omar al-Attrash from Arsal was named as the alleged organizer of a car-bomb attack in Beirut’s mainly Shiite southern suburbs in August that left 22 people dead. Mr. Attrash was killed in uncertain circumstances on Oct. 11, when his vehicle was hit by a missile while driving along a track between Arsal and the Syrian border.

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The residents in Arsal say they have been stocking up on medical supplies and bringing in more fighters from Syria in readiness for expected fighting with Hezbollah in the barren rugged mountains between Arsal and the border.

“Qalamoun is not Qusayr,” says Abu Omar, referring to the Syrian town seized by Hezbollah in June. “The mountains are rugged and we are reinforcing and rearming. We know this battle is coming and we will fight to the last man.”