AARSAL, Lebanon — Set in the foothills along the northeastern edge of Lebanon’s Bekaa Valley, this dusty backcountry town near the Syrian border has seen its population skyrocket over the past two years — a direct result of the raging conflict next door.

Among the town’s population of predominately Sunni Muslims are now Syrian refugees and rebels, including members of Jabhat al Nusra, the Sunni Muslim jihadist group that the United States has designated as a terrorist organization with links to Al Qaeda.

As sectarian violence heats up across Lebanon — in places like Tripoli and Sidon, where fighting has become frequent — Aarsal in many ways illustrates the Syrian conflict's transformation from an uprising against an authoritarian regime into a sectarian civil war with reverberations around the region.

Aarsal has been sporadically targeted by Syrian attack helicopters and artillery over the past two years.

"Everyone wants revenge against Hezbollah," said 19-year-old Fahed, a member the rebel Free Syrian Army’s Farouq Brigade. He was referring to the dominant Lebanese Shia militant and political force, which has been aiding his enemies, the Syrian military.

With a severe gunshot wound in his thigh, three comrades carried him for 12 hours over the mountainous border under constant attack from Hezbollah forces, which had joined the fight in Syria, and finally into Aarsal. Fahed was wounded in the Syrian city of Qusayr, roughly 20 miles north of Aarsal.

Hezbollah's involvement in the fight for Qusayr, alongside its Syrian army allies, further enflamed tensions among Lebanon's Sunni population, much of which supports the rebellion.

"First, we will take our revenge against them here in Lebanon,” Fahel said.

Threats like that turned into a reality Tuesday, when a car bomb ripped through a parking garage in a Hezbollah stronghold in south Beirut, wounding dozens. The bomb reportedly targeted the Islamic Cooperation Center, which is believed to be owned by a senior Hezbollah official.

"Now we are forced to use Lebanese territory in order to liberate Qusayr," one 31-year-old Nusra fighter, who gave his name as Hayel, said from Aarsal in June.

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He had said the group, which is the strongest and best equipped rebel-fighting force inside Syria, would “soon” begin operations against Hezbollah inside Lebanon, but no group took responsibility for Tuesday’s blast.

Many of Lebanon’s Sunni Muslims, which make up about one-quarter of the population, feel they live under the heel of an increasingly hostile Hezbollah.

As an increasingly sectarian war plays out in Syria — with a largely Sunni insurgency battling a regime dominated by members of a Shia offshoot sect — the tensions are playing out in Lebanon, too.

In Aarsal, pro-revolution graffiti is spray-painted on most of the city’s walls. One screed attacks the “donkey regime” of Syrian President Bashar al-Assad.

One man at the epicenter of the conflict here is Sheikh Mustafa Hujeiri, a fiery Sunni fundamentalist preacher who, despite his denials, is widely believed to be close to Nusra leadership.

Unidentified gunmen killed Hujeiri’s brother elsewhere in the Bekaa Valley last month, setting off yet another round of kidnappings and murders in an already volatile east Lebanon.

The Lebanese government’s control over its Syrian frontier is unraveling. Along the lawless fringes of the Bekaa Valley — long noted for its role in the drug trade, kidnappings and organized crime — clashes between the Syrian army and rebels are also frequent.

From a half-built mosque near Aarsal’s town center, the stout sheikh issues clear warnings against the town’s various sects.

According to Hujeiri, “wilayat al-faqih” — or the doctrine of state rule by an Islamic jurist, practiced by Hezbollah’s primary benefactor, Iran — “is a monster that wants to swallow the entire region.”

"Let the Shia and the Christians and the Druze understand that the Sunnis are a force to be reckoned with," Hujeiri said of Lebanon’s various sects, in a heated sermon during a session of Friday prayers in June. "We would rather burn to ashes than be humiliated,” he said.

Just down the road, in the town of Lebweh, some residents blame Sunni militants from Aarsal for the grisly murder of four men, including a member of the powerful Shia Ahmaz clan.

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Mourners gathered to remember Sharif Ahmaz, the clan member — and the Lebanese army has separated Lebweh from Aarsal with roadblocks.

“The state must handle this now,” said Ramez Ahmaz, Lebweh’s mayor and Sharif’s uncle. “We reject sectarianism of all forms.”

But even as a mix of communists, clan elders, Hezbollah members and Baathists, or Arab nationalists, gathered to mourn, Nusra fighters in Aarsal boasted of plans to make this tiny, fragile state a new battleground.

“We are focusing on Lebanon,” Hayel, the Nusra militant, said. 

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