RASBAALBEK — There's a 140-kilometer strip of land on top of the mountains just north of Lebanon's Bekaa Valley that marks the border with Syria. It's here that ISIS has dug in, appropriating a remote corner of Lebanon where the terror group has accumulated militants, resources and hostages to gear up for an impending battle with Hezbollah-backed government troops.

This strip is a strategic area because it allows jihadists from ISIS and the al-Nusra Front, who are allies, to threaten seven small towns in the valley. Among the few who know well the imprecise boundaries of the terror groups' enclave in Lebanon is Talal Iskander, head of the International Red Cross. The organization operates in Ras Baalbek, rescuing those wounded in battle between ISIS and the Lebanese.

"We began work here at the end of 2012," Iskander explains. "The bloodiest phase was last fall when we received around 3,000 wounded from both sides. And the pace has continued with about 60 to 70 injuries a week. Not a day goes by without clashes along the front line between troops and ISIS."

There's a "safe zone" eight kilometers from Ras Baalbek where the Red Cross collects the wounded from both sides, bringing them back to the Sunni town still formally in the hands of the government — though support for ISIS is visible with posters and flags. It's when you leave the northernmost Lebanese town of Arsal in the direction of the mountains that you cross the invisible border with the so-called ISIS "caliphate" led by Abu Bakr al-Baghdadi.

"On the ISIS side, there are small isolated villages, refugee camps, a court administering justice on the basis of Sharia, and other ISIS offices," Iskander says, citing testimony from the wounded who ask him every day to go and help in the zone closed off by soldiers. ISIS has repeatedly made it known that they want to declare the birth of an "emirate" in Arsal to rival the capital of Beirut.

Lying in wait

The opinion of the Hezbollah commanders in Baalbek is that the ISIS "terrorists are shuffling their cards to prepare for battle when the weather changes." Presumably, when the snow-covered peaks thaw, the showdown will begin. ISIS has been preparing, arriving from the Syrian side with militiamen, food supplies and ammunition. Meanwhile, Hezbollah — which considers Bekaa Valley its sacred territory — is ready on the other side for a "cleansing operation" to annihilate the enemy, says a 45-year-old officer who requested anonymity.

Among the the jihadists' trump cards are 30 Lebanese soldiers who have been held hostage since August. Four have already been beheaded. One of these men was Mohammed Hamie, 25, the son of a Shia civil war veteran who had hoped for "bloody revenge against ISIS."

"My son's killers are barbarians, and Lebanon must react as Jordan did after the pilot was burned alive," says Hamie's father Maruf, sitting next to his other three children and a Kalashnikov. "We need to hit them hard, by hanging the terrorists we already have in prison and sending jets to destroy them in Arsal."

Maruf is ready to do his part and says he knows who killed his son. "It was the ISIS Emir to Arsal. It's Hujairi Mustafa also known as Abu Taqie, and he's a dead man walking. He ordered the beheading, and he will be killed, but only after his son dies so he can go through what I did," he says, giving voice to the desire for revenge that's in the air.

Citizen soldiers

Just a few kilometers away, 28-year-old Adel Islaim decided to take up arms and defend his village of Britel from ISIS attacks two months ago. "It was early afternoon when I received a message on WhatsApp saying that ISIS were coming," he says. "I picked up my gun and went into the street. There were 4,000 of us, all the men in the village, and we went to meet them."

The caliphate's militiamen hit the town from above in the mountains with mortars, came down into the valley and killed eight Hezbollah men before they were beaten back. Among the Hezbollah members who rushed to give support to the residents was 27-year-old policeman Mohammed al-Masri. He climbed onto the roof of a house to show us where the attack came from. "We defended with Kalashnikovs, G3 and RPG launchers, but we know that they'll try to come back."

North of Bekaa, Hezbollah is coordinating the defense in front of the mountains. On the other side are the black flags of the caliphate. Further south, the only road leading to Beirut is guarded by soldiers, police and Lebanese intelligence — with dozens of checkpoints. Residents of the Christian village of Ksara feel besieged. "Hezbollah is protecting us because ISIS wants to kill us all," says Munir Dika, a local doctor.

The military is looking everywhere for young people, rented cars or bearded Syrians. They're tracking the car bombs that ISIS has managed to bring, six times now, from Arsal to Beirut — setting them off in the Shia Dahieh district of the capital, which Hezbollah has turned into a bunker where at least 500,000 people live. The outer perimeter is locked and closed by cement boulders, and inside there are large iron bars that can be closed at any time to stop cars entering. The Al Rasul Al Azzam hospital, where Hezbollah takes its wounded, is surrounded by towers and barbed wire.

Shia night patrols guard the outer perimeter of the nearby Sabra and Shatila Palestinian camps to prevent Sunni jihadists from escaping. There is a suspicion that Islamic groups in the countryside are allied with ISIS cells. The armed Shia militants standing in front of Sheikh Hassan Nasrallah's house mark the point where buses leave to go to Syria, and posters of Hezbollah's fallen heroes and martyrs adorn the walls, lampposts and windows.

"This conflict against ISIS is so different from what we have fought before," says Hamza Akl Hamieh, a former collaborator of the late Ayatollah Khomeini who now lives in exile in Paris. He became a military leader of the Amal Movement in Lebanon, best known in Europe for a spate of hijackings between 1979 and 1982. "We have to defend Lebanon from a tribe of murderers," he says. "It will be hard, but they will lose."

Hence the ongoing preparations to regain the strip of land ISIS controls in the Bekaa mountains. The voices from the Beirut suburbs suggest that Hezbollah will have hundreds of soldiers ready to fight, and the orders have been given to "take no prisoners." The veterans of the 2013 battle of al-Qusayr expect fierce clashes. "Those who fight for ISIS are inhuman," one says. "They face bullets without showing any fear."