Amid growing violence between Sunni and Shiite militants across the Muslim world, there are fears this will lead to a showdown between Lebanon's long-antagonistic Shiite Hezbollah, Iran's powerful proxy in the Levant, and Saudi Arabian-backed Sunnis.
A recent eruption of Sunni-Shiite clashes across Lebanon has "heightened the risk of chaos in Lebanon and another conflagration with Israel," warned analyst David Schenker of the Washington Institute for Near East Policy.
The fall of the regime of President Bashar Assad, dominated by Alawites who are an esoteric offshoot of Shiite Islam, would almost certainly produce a successor regime led by Syria's majority Sunnis, repressed since Assad's father seized power in Damascus in a 1970 military coup.
That would leave Hezbollah cut off from its supply routes from Iran, via Syria, the Shiite ally and leave it dangerously exposed between its Sunni rivals in Syria and Lebanon and its longtime nemesis Israel in the south.
Sectarian tensions in Lebanon, which helped ignite a 1975-90 civil war that cost an estimated 150,000 lives, have been rising alarmingly because of the bloodbath that began in Syria, which under the Assads long sought to dominate Lebanon, March 15, 2011.
Lebanon's Sunnis, heavily outnumbered by the Shiites, the country's dominant sect that's more powerful than the national army, have been on the ropes since the Feb. 14, 2005, assassination of former prime minister Rafik Hariri, the Saudi-backed Sunni leader.
Four members of Hezbollah, including one of its chieftains, have been indicted by a U.N.-mandated international tribunal for that killing in a massive bombing in downtown Beirut. Hezbollah denies that. Its leader, Hassan Nasrallah, refuses to hand over the suspects to the tribunal or Lebanese authorities.
Hezbollah's critics say it carried out the assassination with the backing of Syrian intelligence because Hariri, five times Lebanon's prime minister and its leading statesman, was rebelling against Syria's control of Lebanon that began in 1976 during the civil war.
This corrosive sectarian conflict has been growing since that fateful St. Valentine's Day massacre in 2005.
The Saudis, who lead a widening Sunni shadow war against Shiite Iran, have been arming their Sunni allies in Lebanon, seeking to match the Syrian and Iranian arms going to Hezbollah.
Riyadh is also funding and arming the Sunni rebels in Syria, particularly the jihadist Jabhat al-Nusrah that's one of the most effective military groups fighting the regime. Its specialty is suicide bombings.
There are widespread suspicions that reports of a growing jihadist threat in Lebanon have been systematically exaggerated by Hezbollah through its media outlets to push the pro-Assad line that the conflict in Syria is really a battle against al-Qaida.
But whether that's true or not, the Syrian war has, inevitably, been spilling into Lebanon, where Hezbollah and the jihadists send men and weapons into their allies.
There have been numerous violent episodes involving jihadists in Lebanon, where Islamist hard-liners were supposedly crushed by the army in 2007 after an epic 15-week battle in a northern Palestinian refugee camp.
Jihadist cells have been building up for years in Palestinian camps, where authorities cannot enter.
In December, heavy fighting flared in the northern city of Tripoli after 17 Lebanese Sunnis were killed in Syria.
On Feb. 1, this swelling feud erupted in the eastern town of Arsal in Lebanon's Bekaa Valley, Hezbollah's heartland near the Syrian border.
Gunmen killed Khalid Ahmed Hmayed, 43, a known Sunni activist aiding the Syrian revolutionaries. The army says undercover intelligence agents shot him when he resisted arrest. Townspeople say he was gunned down with no attempt to seize him.
Sunni hard-liners pursued the "army unit," cornered them in a snowdrift and killed a captain and a sergeant.
Residents say four other men were killed as well.
"We brought six dead from the mountain but the army only announced two deaths," one resident said.
There are widely held suspicions in Arsal the four were Hezbollah operatives helping the military, heavily infiltrated by Hezbollah, which holds sway in the mountainous region, because of Hmayed's support for Jabhat al-Nusrah.
Hezbollah denies any involvement.