We did leave Syria alone.
The war there started without our help, and it will end without our help. That's a lesson we learned, or should have learned, in Iraq--you can certainly intervene and change things, but you never know how they're going to turn out when you do.
No I'm not saying we should always sit back and do nothing. Sometimes you have to act. But its like starting a fight with a stranger in a seedy bar on the wrong side of town. Sure, sometimes you have to fight if you have any hope of getting out alive at all. But until you're convinced that's your only choice, you're better off just walking out.