I doubt the US is aiming to "invade" Syria. As time goes on, military intervention becomes less and less likely. Obama has even recently stated that military intervention is out of the question. The fact is that Syria's geo-political centrality ensures that no state, especially those with vested interests in the country, want to see anarchy reign. In short they are willing to tolerate human rights abuses and wide-spread supresion of opposition movements, to ensure that Syria dosen't become a failed state.

Syria borders already volatile Iraq, turbulent and secterian Lebanon, Israel, Jordan and Turkey. It is without a question that much of the west and many of these states listed (including most of Lebanon) would prefer a regime more freindly to their interests. But it is clear that neither of these states are willing to risk an already destabilizing situation to escalate even further. After all who wants a safe haven for terrorists, trafickers and criminals right next to them, or in such a key position in the Middle East?

On the other hand their is Russia and Iran who have a different kind of vested interest. Russia has a key naval base, the only outside Russia and their only warm water port. Russia also sells large quantities of arms to Syria. To Iran Syria is the only Arab ally it has. Syria is ruled by a family of Alawaites (an offshoot of Shi'ism). It has helped Iran funel arms, funds and training to Hizb' Allah in Lebanon for decades.

While most of the West, would likely prefer regime change that is not destablizing, it is evident that most states are willing to tolerate human rights abuse in Syria and violence commited by the regime upon the Syrian people, in order to ensure that order remains in the region. Syria is far different to Lybia. It is more densely populated, dosen't have massive quantities of natural resources which the West would like to exploit, Syria is geo-politically centric whereas Libya is off next to Algeria and Egypt, where most of its surface area is covered by uninhabitlable desert, Libya is not next to such centrally key st ates as Israel, Iraq, Turkey and Lebanon.