The more I think about it the more I think that US (or any other Western state) intervention in Syria would be a gift to Iran. That would potentially allow them to play the anti-imperialist/anti-crusader (or take your pick) card and take a more direct role in supporting the Assad regime. I very much doubt that they would risk a full scale invasion but they certainly could be more overt about sending materials and "volunteers" with less risk of a backlash in Sunni Arab opinion. That is a potential catfight we do not need to get dragged into right now (or ever really). The Arab Spring has shown Arabs that these authoritarian regimes can be over thrown. Assad's forces may clear an area but as soon as they leave the rebels come back. That is not a sustainable position for the regime, not indefinitely. If the rebels could somehow convince Syria's minorities, such as the Christians that they would be protected from a Sunni backlash after the fall of the regime it would hugely undermine Assad's support. Unfortunately, that is probably an academic argument. I am not so sure that they can be protected, even if the leadership of the rebellion wants to. Even if they could be protected I am also not so sure that those minority groups could be convinced that they would be protected. The fact that two thirds of Iraq's pre-war Christian population has fled Iraq does not set a particularly encouraging precedent.