Worst case scenario for this probably goes like this:
Syria becomes involved either via an outbreak of conflict in the Golan Heights or through Hezbollah. It then moves troops both into Lebanon and South against the Golan Heights and Israel. It will probably claim to be protecting Lebanese sovereignty from the evil Zionists or something akin to that.
Iran, because it has a mutual defence pact with Syria, send troops, weaponry etc. into Lebanon through Hezbollah and directly to the Syrian armed forces.
Nobody can move ground troops except Israel and the US, for they have air superiority. The only why Syria becomes involved is if Israel goes looking for more targets to hit.