Driving into Tripoli, in northern Lebanon near the Syrian border, is like entering a city under occupation.

People go about their business; shopping, visiting friends and family, but the military has heavily fortified checkpoints at key intersections.

They are backed up by armoured personnel carriers armed with machine guns, stationed down the backstreets, ready to quell sectarian bloodletting, and suppress the jihadists and the stories they tell.

In Bab al Tabane, a poor and dirty neighbourhood known as a centre of radical Sunni militancy, there are many young men who have answered the call to arms in Syria.

One of them, Hassan Srour, a young man in his mid-twenties, agreed to go on camera to detail his brief but gruelling bid to join the fight.

It was the early days of the insurgency, 2012, and his brother, Hussein, had already gone over.

"My brother ... is the one who encouraged me. He would describe what was going on, how they would bomb and kill children," Hassan told 7.30.

These were powerful stories — the sort that drew thousands of fighters into the fray.

"I went because I saw how much they oppressed the people. I saw what they were doing against the children," he said.

While Hassan does not fit the mould of the stereotypical jihadist — he doesn't believe in a caliphate or want to kill Westerners — but he is typical of many who felt a religious motivation to join the fight.

"Our jihad is for God. People give their life, not for money, nor anything else," he said.

"We fight for a precise objective: for the victory of a sect, for the victory of the oppressed."

'I went through four full days of torture'

Hassan Srour was detained and tortured in Syria.

Not long after Hassan was reunited with Hussein in Syria, their rebel platoon walked straight into an ambush by pro-government fighters.

It was a bloodbath and Hassan was the only survivor.

After a couple of days on the run, he was captured and taken into the Assad Government's torture chambers.

"When they caught me I went through four full days of torture," he said.

"The full range of torture: with electricity, beating me in unimaginable ways, with a baton, with a wire."

The killing in Syria's detention centres is on such a massive scale and so systematic that UN investigators say it amounts to an attempt to exterminate the opposition, a crime against humanity under international law.

"The prison had a maximum capacity of 200 but held over 5,000 detainees. Every day, two, three or four people would die because of torture," Hassan said.

"I swear to God, I have seen with my own eyes so much torture. They would torture them with electricity with batons, with just anything."

But Hassan survived thanks to a vocal campaign by his family, friends and hard-line Islamist groups.

They blocked streets and staged protests. And they appealed to the head of Lebanon's powerful General Security apparatus, Abbas Ibrahim, who is a Shi'ite widely seen as allied to Lebanon's powerful Shi'ite militia, Hezbollah, which is sponsored by Syria and Iran and fighting on the side of the Syrian Government.

Incredibly, it worked and Hassan and the dead bodies of his comrades were returned.

He was lucky to be alive and happy to be home. But that is when he saw the video the pro-government men had posted on YouTube after the ambush.

It shows them abusing Hussein's body, and the bodies of the other men.

He showed me the video, sitting in the gloom of a room in the backstreets, his eyes fixed intensely on the screen, embedding another layer of pain and hate.

"They burnt my brother and tortured him with knives when he was dead," Hassan said.

"So, if I meet them, I would do worse. And then people would say I'm a terrorist."

It is obvious that for this young man, and so many like him, the call to combat has barely been quenched.

Tripoli and Syria have a troubled history

Anti-government rebels in Syria. ( Youtube )

Long before the borders of Syria and Lebanon were invented, Tripoli was tied by kinship and trade to Syrian cities like Homs and Hama, just a little further inland.

The conflict in Syria also had a powerful religious pull, pitting rebels from the majority Sunni population against President Bashar al Assad, who is an Alawite, a branch of Shia Islam, and backed by Shi'ite Iran.

And then, as an afterthought, Hassan added a fact that gives a whole new perspective on this fight.

During Lebanon's civil war in the 1970s, when Palestinian militants were active in Tripoli, the Syria Government intervened and stayed in Lebanon nearly 30 years.

In the early 1980s Syrian Islamists rebelled against the government of Hafez al Assad, Bashar's father.

In response, Hafez al Assad crushed them with a brutality that remains infamous to this day.

The trouble spread to Tripoli and Hafez al Assad's army cracked down here too.

Growing up, Hassan's father told him the stories of those days.

"They shot one uncle in his head and assassinated another. He was driving his car, and they killed him," he said.

Against this backdrop, the militancy in Tripoli sparked by the current Syrian civil war nearly spiralled out of control.

It threatened the Sunni elite and risked a conflict with Hezbollah that the Sunnis could not win.

And now the city is in a sort of lock down.

'What Westerners accuse IS of doing, the Syrian regime is doing as well'

After a long day of furtive filming, the sun was setting and we began to head home.

A military jeep came quickly out of a side street.

We pulled over to let it pass. But we had misjudged.

The soldier waved us down, blocked our path, got out and demanded identification.

His commander asked, "You know you are in the Red Zone?"

After a series of telephone and radio calls, we were ordered to leave the city, immediately.

Despite the watchful eye of the military, the conflict across the border is adding another chapter to a long history of bitterness and it won't be suppressed forever.

"The regime is doing the same as the Islamic State group," Hassan said.

"What Westerners accuse IS of doing, the regime is doing as well. Isn't the regime killing, assassinating and executing?"