The Iranian-backed Hezbollah has long been reported to be providing Shiite veterans of its long war against Israel to aid the Syrian regime of President Bashar Assad against its largely Sunni enemies.
Hezbollah has repeatedly denied it's aiding the regime, which is dominated by Syria's minority Alawite sect, a Shiite offshoot. But a steady procession of hometown funerals in Lebanon for fighters slain in Syria indicates otherwise, and there have been persistent reports of sightings of Hezbollah fighters in Syria.
A recent video circulating in Lebanon shows what are purportedly Hezbollah fighters defending an important gold-domed Shiite shrine in south Damascus.
The fighting raged around the tomb of Zeinab, granddaughter of the Prophet Muhammad and daughter of Imam Ali. He founded the Shiite sect in the seventh century in a religious schism that has split Islam ever since.
The video bears a logo of a furled green banner and the name, in Arabic, "the Abu Fadi al-Abbas Brigades."
Security sources say this is a likely reference to a small Iranian-backed Shiite group that carried out attacks against U.S. forces in Iraq in 2005-08.
The Saudis, who want to topple Assad's Iranian-backed regime to halt Tehran's drive to expand its influence in the Sunni-majority Levant, particularly Lebanon, have been funding Sunni groups in Syria, most notably the Free Syria Army, against the regime.
Saudi Arabia, a Sunni kingdom where Islam was born, and neighboring Qatar have been arming Sunni groups since the Syrian war began in March 2011. But security sources in Beirut say the Persian Gulf monarchies are recruiting Sunni fighters in Lebanon to build up the anti-regime forces.
This operation could backfire by igniting sectarian passions in Lebanon that are already dangerously high.
Hundreds of Lebanese Sunnis are reported to have gone into Syria to join Jabhat al-Nusra, one of the dominant Sunni forces that's widely seen as linked to al-Qaida.
Most of these men were recruited in Lebanon's northern port city of Tripoli, where Sunni and Alawite communities regularly battle each other, a confrontation that's escalated since the Syrian war began.
More than 70 people have been reported killed in the Tripoli clashes, as well as smaller-scale eruptions in Beirut, in recent months.
Tensions are also rising in the Sunni-dominated southern city of Sidon.
Lebanese sources say that much of the heaviest fighting in Syria involving Lebanese Shiites and Sunnis is taking place in the central province of Homs. That links Damascus to the Alawite heartland in the northwest around the Mediterranean port of Latakia.
There have been repeated reports that Assad's Plan B, if the rebels seize Damascus, is to retreat with his regime stalwarts, including his inner circle and his generals, into that mountainous Alawite redoubt.
Most of the heavy fighting is currently in Damascus and in the north, where rebel forces have supply lines running from Turkey, another Sunni nation that wants to see the end of the Assad regime.
Rebel strategists say Homs is vital to seizing Damascus itself, and it is in Homs that the decisive battle with be fought.
The regime has managed to cut off several key rebel supply routes from Lebanon. The rebels believe they must regain these if they're to control central Syria and Damascus, so Lebanese fighters are being moved up to the warfront.
The battle for Syria epitomizes the titanic religious and political struggle between Iran and Saudi Arabia that's swelling daily, and the brutal Sunni-Shiite sectarian war across the Middle East and Asia that entails.
Control of Syria is vital for Iran in its confrontation with Israel, Syria's southern neighbor, and for securing access to the Mediterranean.
"From the Saudi point of view, this is a historic opportunity to decisively roll back Iranian encroachments from the Sunni Arab world," observed the U.S. global security consultancy Stratfor.
The Saudis are so consumed with this, Stratfor noted, they're "once again flirting with jihadists and sectarian militias who will not only fight in Syria but will also take the battle to Iraq and help weaken the (Shiite-dominated) political order there."