GWEN IFILL: We turn now to the conflict in Syria and its spillover into neighboring Lebanon. Just yesterday, 12 people were killed on Lebanese soil during a battle between pro-Assad Hezbollah guerrillas and Syrian rebels.
Margaret Warner is on a reporting trip to the region, and tonight she examines the role of Lebanon's fighters engaged on both sides of Syria's bloody civil war.
MARGARET WARNER: High in the Bekaa Valley town of Machgara, Lebanon's Shiite Party of God, Hezbollah, celebrated its resistance and liberation day, complete with martial music, youth scouts, flags, and posters of iconic Shiite figures from Lebanon and Iran, the occasion, the 13th anniversary of Israel's withdrawal from Southern Lebanon after nearly two decades of occupation.
The thousands who gathered here in the Bekaa today came to celebrate Hezbollah's founding ideal: resistance against Israel. But they also heard their leader make the case for why Hezbollah is now adding another front to its long war, that new front, defense of one of its chief allies, the regime of Bashar al-Assad in neighboring Syria, where a brutal civil war has killed more than 80,000 and displaced four million.
Hezbollah troops have helped turn the tide for the Assad regime in the current battle for the strategic town of Qusayr. They have also added to the conflict's ominous sectarian bent, as Assad's Alawite sect, an offshoot of Shiite Islam, confronts a mostly Sunni rebellion bolstered by a fervent jihadist core.
Hezbollah leader Hasan Nasrallah, speaking by videotape, touted his fighters' role across the border, saying they were defending all of Lebanon, Shiites, Sunnis, and Christians, against the fundamentalist al-Qaida-like Sunnis within the Syrian opposition.
HASSAN NASRALLAH, Hezbollah Leader: We regard the control these groups have over Syria and parts bordering Lebanon as a grave danger to all Lebanese. And we have been promised here in Lebanon that this scourge is coming our way.
MARGARET WARNER: Given Assad's longtime supply of money, weapons and political muscle to Hezbollah, Nasrallah argued that, by defending him, his fighters also were protecting their core mission of resisting Israel.
HASSAN NASRALLAH: Syria is the backbone of the resistance. If Syria falls, the resistance will find itself under siege, and Israel will invade Lebanon.
MARGARET WARNER: 22-year-old Hasan Saad responded to that message.
HASAN SAAD, Hezbollah Supporter: We support the participation of Hezbollah in Syria. Bashar al-Assad helped us win the war over Israel. All our weapons were entering from there.
MARGARET WARNER: Grandmother Soukna Hassan shared Nasrallah's fear of the Sunni extremists in the Syrian opposition.
SOUKNA HASSAN, Hezbollah Supporter: If they are not stopped in Syria, they are coming after us. We don't slaughter people. We are not savages like them.
RANDA SLIM, New America Foundation: The main element of Hezbollah's narrative to his constituency is that the fight in Syria is a preemptive war.
MARGARET WARNER: Randa Slim of the New America Foundation, author of a forthcoming book on Hezbollah, says Nasrallah is casting this as a necessary war of prevention.
RANDA SLIM: It's better for us Shias and Hezbollah to fight them inside Syria, instead of waiting for them to come to our backdoors.
MARGARET WARNER: And are Shia here buying that?
RANDA SLIM: So far, the majority of the Shias, and particularly the core constituency of Hezbollah, has bought lock, stock, and barrel into this narrative.
MARGARET WARNER: Not all, however. One of Hezbollah's founders, Sheik Subhi al-Tufayli, says Nasrallah is simply doing the bidding of his patron, Iran.
SHEIK SUBHI AL-TUFAYLI, Hezbollah Founder: The truth is, Hezbollah is ordered to defend the regime. The party knows and all its leadership know that this decision is wrong and destructive, and that it would put an end to the party. Despite this, the party had to accept the Iranian decision. Unfortunately, if things continue this way, we are heading towards a destructive Sunni-Shiite war.
MARGARET WARNER: Tufayli, who split from the group more than 15 years ago, said Lebanese Shiites siding with oppressors like Assad will bring disastrous consequences.
SHEIK SUBHI AL-TUFAYLI: A war this size will provoke the people and cause the deaths of millions. Going to Syria from this country to participate in the war is indirectly an invitation for others to participate in this war.
MARGARET WARNER: By others, he meant Sunnis in Lebanon and the wider Middle East. While most Shias in Lebanon and the region support Assad and Christians are divided, most Sunnis support the Western-backed Syrian rebels.
Radical Sunnis like Sheik Ahmad al-Assir in the port city of Sidon has been doing just that, urging his followers to join the Syrian rebels' fight against Assad. Friday, he cited Nasrallah's speech.
SHEIK AHMAD AL-ASSIR, Sunni: Nasrallah said, I want you to kill the Sunni, who are worse than the Jews, to go fight in Syria to fight the rebels. We consider those words launching war against Sunnis throughout the world.
MARGARET WARNER: Assir, who made a recent show of going to the front lines in Syria himself, concedes Lebanese Sunnis assisting the rebels are no match for Hezbollah's reinforcements for Assad.
SHEIK AHMAD AL-ASSIR: They have more military experience and they have much more developed weapons. We know that our people are not going to make a difference, but it is our duty to send them.
MARGARET WARNER: So do you think this is turning into a Sunni/Shia war?
SHEIK AHMAD AL-ASSIR: I consider this a war launched by those Shia groups who have always carried animosity and hatred to the Sunnis throughout history.
MARGARET WARNER: Hasan Srour of Tripoli, a Sunni, answered the call to help the Syrian rebels. But he and his comrades were attacked by Assad's forces. Most, including his brother, were killed.
HASSAN SROUR, Sunni: I went there because all the Sunnis there are being slaughtered and tortured. This is our religious duty, to go and defend our families there.
MARGARET WARNER: Syrian rebels, like 21-year-old Mayass Tayar, are getting another kind of help from Lebanese Sunnis, like space to build a shelter for his family in Sunni border towns such as Arsal, a haven for fighters from the Free Syrian army.
Fresh from the battle in Qusayr, Tayar said the Hezbollah militias there are deadly and effective.
MAYASS TAYAR, Syrian Rebel: They are wearing military uniforms and their faces are covered. They are many in number, thousands. They have very sophisticated weapons. They even have M-16s, which the Syrian army doesn't have. The Syrian army has Kalashnikovs.
MARGARET WARNER: Sunni Arsal is set in the mostly Hezbollah northern Bekaa Valley, near Shiite towns like Hermel and Al Qasr and the Hezbollah-dominated border area nearby.
We are just inside the Syrian border. And seven miles farther, through the haze, lies the city of Qusayr, where Hezbollah is fighting alongside Assad's forces to retake the town. Hezbollah popular committees used to have to operate from the fields right now, but now with gains on the ground, they have been able to move much deeper inside.
Syrian jets roared overhead as a constant thunder of artillery pounded the city. The Hezbollah escorts who brought us here wouldn't talk to us on camera. But they did take us to meet Syrian soldiers at a nearby Syrian army outpost, showing how close the two were and how to locals like this farmer, the border here is nothing but a line on a map.
Hezbollah's involvement in Syria has come at a price. Shiite towns in the Bekaa Valley have come under increasing rocket attack by Syrian rebels or their sympathizers. Among the most frequent targets, the Hezbollah-dominated town of Hermel.
Vice Mayor Issam Blabel took us to the site of last Tuesday's rocket attack.
VICE MAYOR ISSAM BLABEL, Hermel, Lebanon: The rocket fell at 4:00 in the afternoon, when people are resting in their homes.
MARGARET WARNER: Hermel has been hit with more than 70 rockets in the last month, ever since the battle for Qusayr intensified.
ISSAM BLABEL: Every house here has a Hezbollah member, and the Shia are almost all supporters of Hezbollah. They claim that this is a base for launching operations against them, but they are targeting it because it's purely a Shia village.
MARGARET WARNER: The rebels have said, a couple different groups, that if Hezbollah doesn't withdraw from Syria, that they are going to increase attacks on Hezbollah bases in Lebanon.
ISSAM BLABEL: Our bases and offices are not exposed, and nobody knows where they are.
MARGARET WARNER: Blabel said 3,000 to 5,000 Hezbollah fighters are in Syria. Whatever the number, they are battle-tested from fighting Israel to stalemate in 2006.
Do they operate, under whose command?
ISSAM BLABEL: Under the command of the Syrian army. Since 2006, the Syrian army has been helping the resistance against Israel. They have been supplying us with arms and ammunition and logistics, whatever we need.
MARGARET WARNER: What's new and increasingly routine are Hezbollah bodies being returned for burial from battles outside Lebanon.
We surreptitiously filmed this funeral procession in the Central Bekaa Valley town of Baalbek, honoring two fighters whom they call martyrs killed in the battle in Qusayr. Some 200 are thought to have died so far fighting for Bashar Assad.
Randa Slim says it's unclear how long Hezbollah leaders can sustain that among their ranks.
RANDA SLIM: It's hard to tell how many men will have to die before you have the first mother in black of the fallen standing up in public and saying, I'm not sending my second son or my third son to fight in Syria.
MARGARET WARNER: So far, that mother has yet to make her stand.