I suggest the first questions that need to be answered is wether intervention is necessary and can be justified.
Necessary for what and necessary for whom?

Justified by what criteria and justified for whom?

Pretty much anything can be deemed "necessary and justified" depending on perspective and circumstance. Asking whether it is justified or not is pretty much meaningless without knowing the details and assumptions behind the proposal. Additionally, there's no additional context regarding what "intervention" means exactly? Is it arming rebels? "Humanitarian" corridors? No-fly zones? Unlimited air support for the rebels? (BTW, which rebels?) A ground invasion? Some combination? What is the political goal for the intervention?

There are millions of combinations. I think I could come up with some combination of circumstances that I think would make a US intervention both necessary and justified. That's a fun game, but looking at actual circumstances in the present tense I don't think intervention by the US is either necessary or justified when compared to the costs and risks of intervention. The most I'd be willing to do at this point is assist the rebels.

I'm willing to change my mind, however. The problem is that you haven't presented anything beyond vague notions of doing something along with concern trolling about US impotence. It's easy to declare something necessary, justified and achievable and you've proven yourself quite capable of making such declarations. I can do that too. It's a bit more challenging to develop an actual strategy, much less operationalize it. So here's your opportunity to present your plan to make things right in Syria, however you define it. Doesn't have to be too detailed. Ann-Marie Slaughter did it in about 800 words in the New York Times a few days ago. Should be easy for you in half that. Hell, even a broad outline would be something.