Has anyone looked at a map of the Mideast? If Lebanon is taken out of the picture, Syria has, quite literally, no friends on any of its borders. At least none friendly enough to help them, I'd bet.
Turkey to the North, Jordan to the south. Both are pretty moderate for Muslim countries, and I doubt that they're very happy about Syria's part in plunging the mideast into this war by suppporting Hezbollah.
I doubt that they're fans of Israel, but I think that keeping their ties to The United States is more important to them than is helping Syria. I'd bet that they sit this one out.
Then, of course, they have Iraq to their east, with 130,000 well-armed American soldiers sitting there. I'm sure that we could spare a couple platoons to send over there if the need arose.
And if Lebanon is denied to them, they no longer have a direct link to Israel's border. Blockade their coast and they are cut off completely.