Syrian conflict's sectarian shadow over Lebanon

This is turning into a dark and nervous summer in Lebanon. Peace depends on maintaining a delicate balance of power between different religious groups.

This summer, the formula isn't working. The reason is the war in Syria.

From the beginning, the Lebanese have been deeply divided about what's happening over the border. The regime of President Bashar al-Assad has a long history of meddling in Lebanon; it has allies, and enemies, here.

Even though Lebanon's leaders have taken sides in the war, they have also been aware of how dangerous it is for their own country. But that has not stopped them supporting their favoured sides in the war.

The stresses and strains have been building up, and now many Lebanese agree that the country is getting close to breaking point.

The most powerful military force in Lebanon is Hezbollah, the Shia Muslim militia and political movement. It has a close alliance with Syria and Iran. Over the months there were many reports of covert Hezbollah support for the Assad regime.

But Hezbollah's leader Hassan Nasrallah justified its recent decisive intervention in the fighting for the Syrian border town of Qusair openly and unapologetically. Hezbollah lost many men in Syria, but they've been hardened by years of fighting Israel and, for now, have the rebels on the run.

Mr Nasrallah said he realised that going into Syria would have consequences in Lebanon. But it was necessary for Hezbollah to fight its enemies, and he called for calm at home.

Hezbollah's supporters agreed with him, and precious few others.

Siham Habib's son Hussein was a Hezbollah fighter, who was killed in Syria in April. Full of sadness for her dead son, she said his life was not wasted, because he was fighting men from the Nusra Front, Sunni Muslim allies of al-Qaeda, and the most effective rebel fighters in Syria. She even elevated the fight above the battles with Hezbollah's old enemy.

"We're up against something more important than Israel - the Nusra Front. If we don't take a stand, who will?"

In Lebanon, there are fears that Hezbollah's intervention in Syria has pushed the country closer to another civil war of its own. The sects are not all armed and ready to fight as they were in the 1970s. Hezbollah, which fought Israel to a standstill in 2006, is massively more powerful than all of them.

But even though the Middle East is awash with weapons, prices have risen sharply in Lebanon, showing that demand is strong.

The increasingly sectarian nature of the Syrian war resonates across Lebanon, where loyalty to a sect often comes before loyalty to the nation. Most of the armed rebels in Syria are Sunni Muslims. Some of them are jihadists who support al-Qaeda.

President Assad's power base is rooted in his own Alawite sect, which is an offshoot of Shia Islam, and now Lebanese Shias from Hezbollah are fighting for him. The war in Syria didn't start as a sectarian fight, but it's turning into one.

That matters in Lebanon because the sectarian divisions in this country mirror those in Syria. In recent weeks there have been armed clashes between rival groups in Tripoli and Sidon, and sectarian killings in the Beqaa Valley.

Elias Hanna, a retired Lebanese general turned university professor, summed it up.

"You cannot go as Hezbollah, a part of the Iranian axis and, as a Shia go to Syria and kill the Sunnis and come back and live in Lebanon surrounded by the Sunnis. It will create what we call internal strife in Lebanon.

"And maybe one day will create or revive what we call the security dilemma in Lebanon and everybody will try to arm himself, and maybe we are heading toward a civil war; it's going to be maybe the bloodiest civil war in Lebanon."

A Lebanese man called Ahmed Niami, visiting a group of Syrian refugees in the Bekaa Valley, told me that sectarian tension was infecting every part of life.

"If the conflict in Syria is going to go on longer, Lebanon is going to be in a worse state than Syria is in today. If there's a car accident on the street, people run to ask, are you Sunni or Shia… Sectarian divisions have become very, very deep, and it's going to get worse."

Shia-Sunni tension is on the rise right across the Middle East: the biggest single reason is the sectarian killing in Syria.

What is happening between Shias and Sunnis is becoming the Middle East's defining conflict - the one that crosses borders, the most destructive way that the Syrian war is spreading hatred and violence.