But more broadly, Hezbollah’s deepening involvement shows how the Syrian civil war is exacerbating tensions between Shiites and Sunnis around the Middle East.

Syrian rebels accuse Hezbollah of fighting alongside Assad’s troops and attacking rebels from inside Lebanese territory.

In recent months, fighting has raged in and around several towns and villages inhabited by a community of some 15,000 Lebanese Shiites who have lived for decades on the Syrian side of a frontier that is not clearly demarcated in places and not fully controlled by border authorities. They are mostly Lebanese citizens, though some have dual citizenship or are Syrian.

Before Syria’s uprising erupted two years ago, tens of thousands of Lebanese lived in Syria…

One commander of the Popular Committees said Shiite villages have been repeatedly attacked and some residents have been kidnapped and killed by rebels. He said that prompted local Shiites to take up arms to defend themselves.