Qasr, Lebanon - Abu Ali Haq is a fighter on a crucial battle line in Syria's ruinous civil war, and he's locked in to what he sees as an existential struggle for his village and his kin living here.

Except, Abu Ali Haq is not Syrian and neither are his fellow villagers. They are Lebanese.

With a scarred nose smashed flat, dressed in battle fatigues and chain-smoking cigarettes, Ali Haq explained: "When attacks started around a year-and-a-half ago we formed the popular committees. The men from our village have entered a number of battles. Three have been killed, but many more have been injured."

The border between northern Lebanon and Syria is nondescript at the best of times, but in many places now it has been erased by more than two years of conflict in Syria.

Lebanese villagers are now playing a key role on the Syrian battlefield with the support of both the Syrian Army and the dominant Shia political and military force in Lebanon, Hezbollah.

Syrian Army attacks Qusayr

Ali Haq's village Saf Sufi is among a cluster of communities that technically fall within the Syrian border, but because of a misguided colonialist's pen stroke in the 1920s they are inhabited by Lebanese citizens. Some 15,000 Lebanese Shia have lived for decades on the Syrian side of a frontier, which is not clearly demarcated or patrolled.

The predominantly Shia residents of the villages have taken a stance against the increasingly Sunni-led Syrian rebellion, and claim they have been attacked and laid siege to by Islamist fighters.

"I'm not interested in this party or that party, but I have the right to defend my home and my family, and I will take support from whoever will offer it," said Ali Haq.

Hezbollah intervention

While Hezbollah ostensibly only supports the Lebanese popular committees from a handful of villages, an extended network of its fighters share in their battle in villages, hamlets and farmsteads at points along an 80-kilometre stretch of borderland.

"We assist them materially and politically, with money, medicines, resources … whatever they need," said a local Hezbollah member, who asked not to be named because he was not authorised to talk with the press.

The support given to the popular committees in the northern border region is reflective of the village guards that Hezbollah used to devastating affect against the Israelis in south Lebanon during the 2006 war.

Hezbollah's full time and extremely well drilled cadres provided training, organisation and arms to local volunteers who wreaked havoc among the invading Israeli ground troops.

"They are again doing what they do best - organising local defenses. I saw with my own eyes that the level of proficiency they displayed in 2006 was amazing," explained Timur Goksel, a former senior political advisor to the United Nations in southern Lebanon.

While Hezbollah is the dominant military and political force in the area, it is perhaps the tribal and family relations between the fighters on the Lebanese and Syrian sides of the border that most solidly cement their loyalty.

"If my cousin is in need of help, I will give him weapons and teach him how to use them. I am trained after all," said a Hezbollah fighter who gave his nom de guerre as Abu Ali. "This is a tribal area, and there is no way we can abandon our kin."

A member of a Lebanese pro-Syrian popular committee [AP]

Providing cover

In the plains of the northern Beka'a valley where the state is almost completely absent, Hezbollah provides security and political cover for the tribes. "When the shit hits the fan they are all Hezbollah," explained Goksel.

While Abu Ali Haq and some of the local Hezbollah fighters were drinking tea and discussing their right to defend the border villages, thunderous explosions boomed from the predominantly Shia and Hezbollah controlled town of Hermel, 5km back into Lebanese territory.

The Syrian opposition's Free Syrian Army (FSA) had targeted Hermel with four Katyusha rockets from several kilometres east along the border.

"I didn't flee when the Israeli's bombed us in 2006, despite six members of my family being injured, and I'm not leaving now," said villager Yousseff Mattar as he inspected the still-smoldering tail of one of the rockets that careered into the hillside several hundred metres from his house.

Immediately before meeting its ultimate fate in the rocky outcrop, the missile had flown directly over a Ferris wheel and family play park.

The FSA launched their first missile attack on the Shia towns of north Lebanon resulting in two civilian deaths, including a 13-year-old boy. Intermittent and sporadic attacks have continued in reprisal for Hezbollah's expanding role in the Syrian conflict.

"These strikes from the FSA are a warning shot to Hezbollah saying they need to rethink their strategy in Syria," said Imad Salameh, a political science professor at the American University of Beirut. "If Hezbollah intervenes in the Syria conflict, then the FSA will be able to bring the fighting into Hezbollah's own backyard."

Battle for Qusayr

The extent of Hezbollah's involvement in supporting Assad's forces is not clear, although it has offered assistance to Shia villages under threat, as well as protecting Shia shrines in Damascus.

The reason Hezbollah's buttressing of the border is causing such consternation among the Syrian rebels is because it is tipping the balance of power in the government's favour in the battle for the town of Qusayr, 8km into Syrian territory.

Control of Qusayr and the surrounding areas is crucial for the opposition fighters to smuggle weapons and men from Lebanon into Syria. For the Assad regime, Qusayr is a vital route north from Damascus to its stronghold in the coastal regions.

After being in rebel control for the past year, a Syrian government offensive was launched Sunday and fighting continues to rage for the strategic town. A large number of Hezbollah fighters were reportedly involved in the attack with more than a dozen killed in the battle.

As it gets further entangled in the Syrian quagmire, Hezbollah will also need to be careful to avoid an escalation that leads to fighting spilling further into Lebanon. Such a scenario would damage its reputation as a purely anti-Israeli resistance movement, and could unsettle its dominant grip over domestic politics.