It’s difficult to overstate how dangerous this new phase is. At the moment, the conflict in Syria is a war of attrition, essentially a contest between the country’s Sunni Muslim majority and its Alawite-dominated government and military. Each side is doing its best to butcher the other, but neither appears to be prevailing. A massive intervention by Hezbollah would obviously be aimed at tipping the balance in Assad’s favor. Indeed, it was no coincidence that Nasrallah decided to give his speech during a big battle for the Syrian city of Qusayr, where Hezbollah appears to have suffered heavy losses. Qusayr lies on the road between Damascus and the Syrian cities on the Mediterranean coast, the stronghold of the Alawites, the minority sect that is loyal to Assad. For obvious reasons, the Assad regime in Damascus wants to hold the highway to the coast. It’s unclear how much a difference a new infusion of Hezbollah fighters will make, but it can’t hurt…

Hezbollah isn’t the only group that has been intervening in the Syrian civil war. Since the Syrian conflict began, Lebanese Sunnis have been slipping across the border to support the rebels, but in a mostly unorganized, haphazard way. The Syrian rebels themselves have promised to avenge Hezbollah’s activities by taking the fight into Lebanon. The most dramatic example of the Syrian civil war’s effect on Lebanese politics came in March, when Prime Minister Najib Mikati resigned. Mikati, a Sunni, was in a coalition with Hezbollah. It doesn’t take much to see how difficult it is in Lebanon today for a Sunni politician to work with Hezbollah, whose fighters are killing Sunnis across the border.

What comes next? So far, the peace in Lebanon has mostly held, in no small way because memories of the civil war there are still fresh. But as Hezbollah commits itself more deeply to the Syrian war, the more difficult it will be to contain the violence in Lebanon itself. It’s not difficult to imagine Lebanon slipping into a new civil war of its own.