BIREH, Lebanon — Sunni Arab leaders, in dark suits or long robes, were pulling up to the mosque in this north Lebanon village, to mourn the death of Sheikh Ahmed Abdul-Wahid. Huge banners, splashed with the portrait of the popular cleric, lined the narrow road to the mosque. One, pasted on the front of the mosque, bore this inscription: “Your blood will not be wasted.”

Most of the mourners believed Abdul-Wahid had been shot dead at a checkpoint by Syrian agents because he openly backed the rebels who are challenging President Bashar al-Assad. They believed his murder was a warning not to back his opponents.

“The Syrian regime wants to spread their civil war to Lebanon,” fumed Khalid Daher, a member of parliament from northern Lebanon. “They want to spread the chaos.”

Indeed, if the Syrian uprising drags on for a second year without any resolution, it could trigger explosions inside neighboring countries — sparked by Assad’s machinations.

The murder of Abdul-Wahid makes clear how pressing that danger is.

To understand the danger, it’s necessary to grasp the roots of the Syrian uprising: It began as a revolt of a young generation against the Assad family dictatorship, which has lasted more than four decades. But that rebellion pitted a Sunni majority against rulers who come from Syria’s minority Alawite sect, an offshoot of Shiism. The Assad family has packed most high posts with its coreligionists who are wedded to the regime.

As Assad’s forces slaughter more and more Sunni civilians — and massacre children in Houla — the conflict has increasingly taken on a sectarian hue. So far, Sunni revenge killings have been limited, says Human Rights Watch’s Nadim Houry in Beirut, “but there is clearly an increase in sectarian tension. We don’t have evidence yet to talk of ethnic cleansing, but it feels like the country is slipping into more and more of a civil war.”

Meantime, in Lebanon, the Sunni minority — based heavily in the north — has seethed in frustration at the political power of Hezbollah, the Lebanese Shiite movement backed and armed by Syria and Iran. As Sunnis rise up in neighboring Syria, Lebanon’s Sunnis have been eager to back them, funneling money, medical supplies, and small arms across the border to the Syrian rebels.

Sunnis view the sheikh’s murder, and a string of other arrests, killings, shoot-outs, and kidnappings, as a dark Syrian message: Assad can upset Lebanon’s fragile sectarian balance if the Lebanese cross him.

Sure enough, Abdul-Wahid’s death was followed by a bloody clash in northern Lebanon’s largest city, Tripoli, between a minority Alawite neighborhood and a Sunni district next door.

Daher told me that Lebanese politicians from all sects, who remember their brutal civil war from 1975-90, are determined to tamp down these sectarian tensions. But should Syria up the ante — say, by assassinating a leading Lebanese politician, a tactic for which its intelligence agencies are noted — all bets would be off.

Moreover, the atrocity at Houla, and any future repetitions, could finally trigger a cycle of revenge killings inside Syria that would echo elsewhere. One of Syria’s other neighbors is Iraq, where the wounds of a bloody Shiite-Sunni civil war have not healed yet. If Syria’s Sunnis become driven to seek revenge on the Alawites as a community, rather than just on the Alawite leaders, their civil war could reignite tensions among Iraqis.

Bottom line: The need for a formula that ends the Assad regime has never been greater. But such a formula is nowhere in sight.

The diplomatic efforts of the U.N. envoy to Syria, Kofi Annan, are going nowhere. He was humiliated by the Houla disaster. Yet military aid to a fractious rebel movement offers no quick path to Assad’s ouster, especially since he still enjoys strong Russian and Iranian support.

However, the prospect of a growing Syrian sectarian war should focus U.S. and Russian minds on the need for coordinated action. The Russians say the demise of Assad would be likely to produce a radical Islamist regime. But unless Moscow presses him to accede to a political transition, the Russians’ reluctance will produce what they most fear.

“The amount of killing is leading people (in Syria) to be more radical in their religious belief and how they perceive other sects,” I was told by Saleh El Machnouk, a prominent young lecturer at Lebanese American University who is well-acquainted with the rebellion.

The Obama administration recognizes this danger. “The longer this goes on, the more extreme it’s going to get, and the greater negative effect it will have on the region and our interests,” said Deputy National Security adviser Denis McDonough on Friday at the U.S.-Islamic World Forum in Doha, Qatar.

He is correct. But the United States will have to move past the Annan plan, and work more forcefully to help the Syrian opposition if this outcome is to be avoided.

When I talked to young Sunni men at Abdul-Wahid’s funeral in Bireh, they were angry. They said they had wanted revenge but for now were willing to let their leaders handle the problem.

How long they, and their Syrian compatriots, will restrain those instincts for revenge is anyone’s guess, especially if Assad continues killing civilians.

And he will.

Trudy Rubin is a columnist and editorial-board member for the Philadelphia Inquirer.