Lebanon's army and members of the Islamic State group and the Al Nusra Front fought each other on Wednesday.
The clashes in Arsal began on Saturday after Lebanese security forces arrested a commander popular with local rebels who often move across the porous border with Syria.
That ended a ceasefire in the most serious example of the Syrian civil war spreading to Lebanese soil.
Arsal is the first stop for many civilians fleeing the bloodshed in Syria. Refugee camps in Arsal that provide shelter to tens of thousands of Syrians have been badly damaged by the fighting, forcing refugees to seek shelter in the town itself.
Saudi Arabia pledged $1bn to help the Lebanese army to bolster security.
So, how serious is this threat to Lebanon?
Presenter - Sami Zeidan
Elias Hanna - a former Lebanese General.
Kamel Wazne - political analyst and founder of Center for American Strategic Studies in Beirut, a think tank that covers regional issues.
Fawaz Gerges - chairman of Contemporary Middle Eastern studies at the London School of Economics and author of 'The Rise and Fall of Al Qaeda’.