Lebanon's border with Syria is growing ever tenser as arms flow one way and violence spreads in the opposite direction.
Lebanon forced into a conflict it does not want
During the colonial era, France tried and failed to divide Syria into six separate states, in part based on sectarian lines. Of those six theoretical states, only one - Lebanon - managed to remain a distinct country and, over the subsequent nine decades, it has always paid the price of foreign entanglements. After skirmishes that started in Tripoli on Sunday, it is clear that Syria's troubles are again bleeding over the border.
Of necessity, Lebanon's varied communities have become adept at patching up quarrels. Since the civil war, sectarian tensions have always haunted politics, but the Taif Agreement that apportions political offices based on sectarian lines preserved, for the most part, a tenuous peace.
That is no mean feat given the circumstances. The sectarian groups that are increasingly polarised in Syria - Sunni and Christian, Alawite and Druze - have direct links to their coreligionists in Lebanon. Beirut has been acutely aware of its vulnerability given the current unrest.
In an interview at the beginning of the year, Lebanon's prime minister, Najib Mikati, told The National: "I am more concerned about what's going on in Syria because it has a direct reflection on Lebanon. What I'm trying to do and my policy is to disassociate, to shy away from what's going on in Syria." Even Hizbollah, which formally supports the Assad regime, has walked carefully when it comes to the Syrian quarrel.
For almost a year, tens of thousands of refugees have been spilling into Lebanon. It is perhaps unsurprising that armed hostilities would follow. On Sunday, Syrian regime soldiers reportedly chased rebels across the border in "hot pursuit" incidents. Four were killed and 20 wounded in subsequent clashes between Alawites and Sunnis.
The long border has grown increasingly tense as munitions pour across it to the Free Syrian Army. The flow of weaponry, widely believed to be coming from sponsors within Saudi Arabia and Qatar, has brought protests from Damascus and an increasing militarisation of the border.
Lebanon's resilience in the face of provocation is well known. But Syria's struggle always posed a serious danger for its neighbours, and particularly for Lebanon where it wielded de facto control for so many years.
There are home-grown reasons why Lebanese will take sides in this conflict; many Alawites, for example, still profess admiration for the Assads, having very few other choices available. Hizbollah's military dominance outside the state institutions continues to distort politics. Until now, those groups for the most part showed restraint.
Arms are being smuggled across the border, and it is perhaps inevitable that violence will flow the other way. History gives every Lebanese a reason not to be drawn into those currents.