The implications for Lebanon are potentially serious: A sustained and significant involvement by Lebanese parties in the Syrian conflict would risk exacerbating a tense situation. If unchecked, such activity could lead to an “uncontrollable situation” in Lebanon, said Imad Salamey, an associate professor of political science at the Lebanese American University in Beirut.

Paul Salem, director of the Carnegie Middle East Center in Beirut, said that while Hezbollah was clearly involved in Syria, the military strength of the organization could limit the repercussions in Lebanon. “There’s no capacity in Lebanon to confront them,” Mr. Salem said.

Still, “if two or three years down the line the rebels finally prevail, then Hezbollah would have been part of the civil war and the rebels may take the war to Lebanon in one way or another,” he added.

Mr. Salamey said that for now, Hezbollah’s role is probably limited to special operations, technical assistance and security along some border areas.

A larger intervention, he said, would be undesirable for the organization and harmful to its strategic standing in Lebanon. But “having Hezbollah play a role in undermining the ability of the Syrian resistance to cross and move freely along the border provides the Syrian regime with significant backing,” he said. “Otherwise, the Syrian Army would have to fight so many wars.”

At various points along the border, which is crossed by vital supply routes for some Free Syrian Army units, the war is a reality on the Lebanese side of the demarcation line. The village of Masharih al-Qaa — a string of farmhouses stretched along rough dirt tracks skimming the border — is one.

In Masharih al-Qaa, the hiss of rocket fire, rattle of automatic weaponry and thud of artillery continuously puncture the night. By day, shelling continues and helicopter gunships circle high overhead.