Several hundred Lebanese Shiite fighters played a key part in Sunday's capture of Qusair, which commands the vital supply route between the Syrian capital Damascus and the Mediterranean coast.
Qusair, 6 miles east of the Lebanese border, had been in rebel hands for more than a year and was one of their last strongholds in central Syria. It's been under attack for weeks. For the last few days the city's been surrounded by the regime's army and Hezbollah fighters engaged in their most significant role so far in the 2-year-old civil war.
Last week, Assad's forces, who controlled areas east and south of Qusair, recaptured several key villages north of the city.
Hezbollah, which reportedly has 2,000-2,500 fighters in Syria, held the ground west of the city.
Recent Syrian army reinforcements included the elite 4th Armored Division, commanded by the president's younger brother, Maj. Gen. Maher Assad.
Hezbollah used multiple rocket launchers and mortars to bombard rebel strongholds in Qusair, while Syrian troops used tanks, artillery fire and airstrikes.
One report Monday said 40 Hezbollah fighters were killed and dozens wounded in the fighting, possibly the highest single Hezbollah toll of the war.
Many observers are convinced Hezbollah's steadily growing profile backing beleaguered Syrian President Bashar Assad, a crucial ally of the Lebanese movement founded by Iran in 1982, against rebels led by Syria's Sunni majority, makes a sectarian showdown a certainty as the war spills over into Lebanon and Sunni-dominated Jordan and Turkey.
The Shiites' main enemy will be the Sunni jihadists who have exploited the struggle to topple Assad and turned it into a sectarian conflict.
The rebels are backed by Saudi Arabia which is locked in a confrontation with its regional archrival, Shiite Iran, for mastery of the oil-rich Persian Gulf and the Muslim world.
In Lebanon, Sunni hostility toward Hezbollah, the most powerful force in the country, has been building for years because of its relationship with the Assad dynasty and Tehran.
That animosity sharpened dangerously in February 2005 when former Prime Minister Rafik Hariri, Saudi-backed leader of Lebanon's Sunnis, was assassinated in a massive suicide bombing in downtown Beirut.
A United Nations-mandated investigative tribunal indicted four members of Hezbollah, including a senior security chief, for the killing. Hezbollah has repeatedly denied involvement and refuses to hand over the suspects.
But most Sunnis blame the group and the Syrian regime for killing Hariri, Lebanon's leading anti-Syrian activist, and other key figures who supported him.
For them, what's happening in Syria now is payback.
Lebanese analyst Hanin Ghaddar, editor of the Now Lebanon website, says tension between Lebanese Sunnis and Shiites "has reached unprecedented levels. Many Lebanese Sunni groups are also moving to Syria to fight alongside the rebels.
"What's happening is that the Lebanese Sunni-Shiite war is already taking place but in Syria."
Assad's most dangerous foe in Syria is the al-Nusra Front, a jihadist group spawned by the murderous al-Qaida organization in Iraq.
As far as is known it hasn't fought a face-to-face battle in Syria. But observers sayit's just a matter of time.
Al-Qaida and its allies, like Fatah al-Islam -- which fought, and lost, a 4-month battle with the Lebanese army in 2007 -- have been gaining strength in Lebanon, particularly in the Palestinian refugee camps that are off-limits to Lebanese security.
The lack of effective Sunni leadership in Lebanon since Hariri's death has driven many of his people to more extreme Sunni chieftains as the crisis deepened.
"Lebanon will become al-Nusra's alternative battlefield," Ghaddar predicted.
"There are no state institutions to control their growing presence in Lebanon or the spread of arms."
On Saturday, Mohammed al-Shalabi, leader of Jordan's al-Nusra Front, declared war on Hezbollah, "our jihadists' main target." They see Hezbollah, not Assad, as the No. 1 enemy of Sunnis.
It's worth remembering that Abu Musab al-Zarqawi, the notoriously savage Jordanian jihadist who created al-Qaida in Iraq, unleashed a vicious and unremitting campaign of blood against Shiites before he was killed in a U.S. airstrike June 7, 2005.