Sunnis and Shiites from Lebanon are streaming into Syria to take up arms on opposite sides of a fierce battle over a rebel stronghold - a fight that has effectively erased the border between the two countries and underlined how Lebanon is being sucked into the civil war next door.
The northeastern Lebanese town of Arsal, dominated by Sunnis, has become a key logistical base for the Syrian rebels who have been fighting for months to keep their hold on the strategic Syrian town of Yabroud, only 20 miles away across the border.
On a recent day, armed fighters in pick-up trucks and on motorbikes were seen scrambling down dusty roads out of Arsal into the mountains to cross into Syria and head to Yabroud.
Syrian rebels move freely back and forth across the border, and rebels wounded in the battle are brought to Arsal for treatment in clandestine hospitals.
At the same time, Lebanese Shiite fighters from the Hezbollah guerrilla group are crossing into Syria to fight alongside the forces of Syrian president Bashar Assad that have been besieging Yabroud since November.
For the past three years, Lebanon has been struggling with the spillover from Syria's civil war.
Sectarian tensions in Lebanon have escalated, as its Sunni community largely supports the mainly Sunni Syrian rebel movement, while its Shiites back Assad.
Hezbollah, the most powerful armed force in Lebanon, has thrown its weight behind Assad, sending fighters who have tipped some battles in the government's favour.
The violence has blown back into Lebanon itself, with suspected Sunni extremists carrying out a string of retaliatory bombings against Hezbollah-controlled Shiite areas.
Around Arsal, all sides are brought into dangerously close proximity, exacerbated by the battle raging just over the border.
The town's Sunni population strongly sympathises with Syria's rebels. Lebanese security officials say a few hundred Lebanese Sunnis are believed to be offering logistical support or fighting alongside the rebels, particularly in Yabroud.
But Arsal is surrounded by mainly Shiite towns in Lebanon's eastern Bekaa valley, raising the potential for friction between the various fighters on Lebanese soil. The town of Baalbek, 20 miles to the south, is a source of many of the Hezbollah fighters heading to join the Yabroud battle.
Syrian rebels being treated at Arsal hospitals said Hezbollah guerrillas make up the bulk of the forces besieging Yabroud.
"They have many weapons, and they are fighting hard because Yabroud is important for them," one rebel, who spoke on condition he be identified only by his first name, Basel, said. "But it's our country and we are strong men. We will defend our people, our land and our honour until we die."
Basel was seriously injured in the groin and left thigh when he and four other rebels were preparing to ambush pro-government forces at Yabroud but were instead ambushed themselves by troops who descended on them from behind.
The 27-year-old needs surgery that Arsal's makeshift hospital, attached to a mosque, cannot provide. But his brother, standing at his bedside, said he will not send him anywhere in Lebanon outside Arsal because he fears he could be captured on route by Hezbollah fighters manning several checkpoints in a neighbouring Shiite village.
"I am going to pay more money to bring doctors here to help him, but he's only leaving this bed to go back to Syria," the brother said. He declined to give his name for fear of reprisals.
Another wounded Syrian rebel, Mohammed Awad, was barely out of the operating room when he began pleading with doctors to let him go back to the front at Yabroud.
The 20-year-old was wounded when a rocket hit a vehicle carrying him and other fighters. His face bandaged after doctors removed shrapnel from his jaw and left hand, Awad said he was determined to rejoin the battle because he is originally from Syrian town of Qusair, another border town that was a rebel stronghold until Hezbollah fighters helped overrun it last year in their first major incursion in Syria's war.
"This is enough reason to want to fight Hezbollah and Assad to death," Mr Awad said.
But there is the issue of personal revenge too, he said: he lost four uncles, two cousins and four female relatives amid the fighting in Qusair.
The battle for Yabroud is particularly fierce because the town is key for rebels. It is their last stronghold in Syria's Qalamoun region, between the Lebanese border and the Syrian capital Damascus, an important route for smuggling supplies to rebels from Lebanon.
Government forces have taken a string of other rebel-held towns in the area in the past month and are now making a final push on Yabroud. Earlier this week, Syrian helicopters attacked the town's outskirts with barrel bombs - containers packed with explosives and fuel that the government has used to devastating effect in other rebel-held urban areas in Sryia.
The fighting has contributed to a wave of refugees fleeing across the border to to Arsal. In the past two weeks alone, 13,000 arrived in Arsal, which has already been overwhelmed by Syrians settling in makeshift camps in the fields and hills on its outskirts.
Facilities for the rebels have geared up as well in Arsal. Two months ago, a new hospital opened in the town with two operating theaters, an emergency room and seven doctors on staff, including several surgeons, who perform an average six operations a day.
So far, up to 200 people have been treated there, mostly Syrian fighters and civilians, said Bassem Faris, a Syrian doctor and the hospital's manager. He was previously in Yabroud treating fighters at a makeshift hospital but had to flee the area after the fall of Qusair.
"Every one of us has a role to play in this revolution, and I will be more useful if I treat people and save lives," Dr Faris said.