1st, Syria doesn't have squat for oil. So if we invade them we can't get paid on the back side.
2nd, while Syria probably has chemical weapons, unlike Iraq they haven't used them on anybody. Nor do they appear to have handed any over to the Hezbollah. So they are aware of consequences.
3rd, while they did invade Lebonnon they did establish some order over complete chaos.
So all in all, they are less of a threat than N. Korea. My guess is we are just going to intimidate them into line, rather than shooting it out with them.