ON THE LEBANON-SYRIA BORDER — As the fighting in Syria intensifies, many Lebanese fear that the conflict could spill over the border, upending the fragile sectarian balance holding their country together and sparking another bloody internal conflict fueled by regional powers.

Already, clashes have erupted along the border in recent weeks, causing alarm among Lebanese politicians and the public that Syria’s army might escalate military attacks against the country for sheltering opposition rebels, potentially drawing in other regional powers such as Iran and Saudi Arabia.

“The violence is becoming worse and it’s becoming more complicated,” a former senior security official in Lebanon said, speaking on the condition of anonymity because of the sensitivity of the subject. “There will be more of a Syrian intervention if the rebels increase attacks.”

The possibility that the Syrian conflict could spread across Lebanon’s borders, and further destabilize the entire region, is also worrying to the United States.

“The United States also remains concerned that the Syrian regime’s use of violence against its own people is contributing to instability in Lebanon,” Deputy Secretary of State William Burns said on a visit to Lebanon in mid-July. “We stress again the responsibility of the Syrian regime to respect Lebanon’s sovereignty.”

A July 21 cross-border attack was only the latest and most dramatic incident along the border, with the Syrian military shelling and shooting into Lebanese villages that it said are harboring Syrian rebels, leaving at least a dozen people dead and many more wounded since May.

That day, Syrian rebels launched an assault on the military in the village of Joussieh and dashed across the border into Lebanese territory. The Syrian military also came barreling over with some 30 soldiers. Fifteen Lebanese were wounded in the raid, and one house was burned down by the Syrian troops.

“The Syrians accused the Lebanese of helping the rebels. They came in shooting and raiding houses,” said Wissam, a resident of the area who spoke on the condition that his full name not be used because he feared for his safety. “The Lebanese army sees all the violations but they don’t interfere because they want to avoid confrontation.”

That attack stirred up public outrage among opponents of Syria’s government here. Lebanese President Michel Suleiman asked the Foreign Ministry to send an official complaint letter to the Syrian ambassador in Beirut, who some Lebanese said should be expelled.

But the complaint got tangled up in sectarian and regional alliances, a common feature of Lebanese politics. Lebanon’s foreign minister, Adnan Mansour, is a member of Amal, a Shiite political party that is a strong supporter of the Syrian government, and the letter he ultimately sent to the ambassador fell far short of a formal complaint.

History and memory

The two countries share a complex and troubled history. For years, Syria has treated Lebanon like a province rather than a neighboring country. Syria’s military entered Lebanon ostensibly as a mediating force in 1976, shortly after the beginning of the civil war, but didn’t leave for nearly three decades.