ARSAL, Lebanon — The patchwork of Sunni Muslim and Shiite villages arrayed along the northern border with Syria are heavily embroiled in the protracted struggle there, but with a distinctive twist.

Fighters from Hezbollah, the militant Lebanese Shiite movement, cross the frontier to fight for Syria’s president, Bashar al-Assad, who is Alawite and whose sect dominates the government. Sunni Muslims sneak over to join the opposition. Once back home in the Bekaa Valley in Lebanon, however, both sides observe an uneasy truce.

“Inside they are slaughtering us, but as soon as we cross into Lebanon there is nothing between us,” said Abdullah, 22, a stocky Sunni farmer who now toils as both a fighter and a smuggler, using only one name to protect his identity. “I would say it is something normal to fight on the other side, given that we are against the regime while they are with it.”

Yet the confrontation over controlling the strategic border throws off sparks that could ignite a bigger conflagration given that it is part of the Sunni-Shiite contest to dominate the Middle East. “There is already a kind of chaos along the border which neither Lebanon nor Syria fully controls, so there is a fear that it will spread into Lebanon,” said Talal Atrissi, a Lebanese academic and expert on Arab-Iranian relations.