In a way, Syria is a proxy for Iran, but not without its own risk: Russia. The discussion here goes beyond the traditional relation between the two countries and reaches, for example, strategic levels. In short, it's about GAS.
1) Russia's prominence in the world follows the prices of natural resources, especially energy;
2) UPI reports that
The U.S. Geological Survey reported in 2010 the Levant Basin, covering all these territories, contains at least 122 tcf of gas and 1.7 billion barrels of oil.See the image below for a map of the area:
So, what's the big deal? I will not go into the details of how the US is yet again invoking reasons that fail to win support for an intervention even at home, that is, an enterprise whose risk-reward profile/opportunity cost keeps even the British away. The big deal here is in calculating the reaction of a revanchist Russia, which may well feel like interwar Germany.
As a side show, the reaction of the Syrian elites surrounding Assad will be interesting. I think, after the lessons in Iraq, Libya, Egypt..., local elites would think twice before turning against their strong man, lest they are left to their own devices to earn a living under market conditions.
So, strategically, the US would probably welcome another failed state. Tactically, the whole question is how remotely from the reactive Syrian capabilities can the US strike? Distance being inversely proportional to effectiveness and efficiency. One can only hope that the Syrian communication capabilities have been deciphered for good and their control capabilities will be scrambled. Shortly, we'll get to infer the Russian state of the art.