Tripoli, on Lebanon’s northern coast, witnessed street battles between pro- and anti-Assad groups back in February. Violence erupted again over the weekend, and intermittent fighting continued for a third day today. Rocket-propelled grenades and machine-gun fire echoed through part of the city. Perhaps five people have been killed, and dozens wounded. Lebanon’s Daily Star carried ominous reports (“Tension and fear gripped Tripoli Monday after both political and security efforts failed to maintain a cease-fire”) and photos of men carrying rocket launchers through deserted city streets and ducking into alleys with AK-47s and Lebanese Army tanks rolling past apartment blocks.This is the clearest sign so far that violence from Syria’s civil war will spill into Lebanon, where Butcher Assad has many allies: some in the national government, others in the “official” security services, still others in community and religious groups, and, of course, Hezbollah.But sympathy for Syria’s rebels, most of whom are Sunnis, is also strong in Lebanon, and particularly in Tripoli. Fugitives and refugees have poured into the city from Syria, and some Tripoli community leaders have been active in supporting Syria’s rebel forces. One such community leader, Shadi Mawlawi, a member of a prominent Tripoli Islamist family, was arrested last week by Lebanese security forces, which might have sparked this weekend’s fighting that centered on communities along Tripoli’s Alawite-Sunni faultline. Worryingly, Prime Minister Najib Mikati condemned Mawlawi’s arrest, suggesting the security services are operating outside his control.After decades of violence throughout the country, clashes periodically erupt in Tripoli between combatants including Palestinian militant organizations; Sunni, Shia, and Druze communities; the national army; and others. That makes for a dangerous tinderbox as the Syrian civil war escalates: Reports are emerging that rebels killed twenty-three soldiers in heavy overnight fighting as the Syrian regime sought to regain control of the rebel-held city of Rastan. Meanwhile, Iraq has so far avoided being sucked into Syria’s fighting, despite hosting over 3,000 refugees at last count.The worst case scenario is not difficult to envision: the conflict in Syria reignites civil war in Lebanon and merges with sectarian violence in Iraq to destabilize the Fertile Crescent from Beirut to Basra. Given Turkey’s concerns with the Kurds in this region and the religious divisions inside Turkey itself, Istanbul would have a hard time staying out of this conflict. Tehran also would feel a strong pull to engage. The United States on both humanitarian and geopolitical grounds might also be pulled into a conflict of this kind.All the more reason to hope for a swift resolution of the crisis in Syria, and all the more reason for concern that an end to that crisis is nowhere in view.