However, I am not convinced that Russia will do much beyond supplying weapons in the event of the US attack on Syria or even Iran. Syria is definitely not worth risking a military confrontation with the US. I think Putin is talking tough because he correctly senses Obama's weakness and indecision in foreign policy matters and hopes that tough talk will make O blink first, but once the bombs start falling, it is an entirely different story.
Russia did not do much to stop NATO attack on Yugoslavia, which was a far bigger threat to Russia than an attack on Syria. Russia's tactical position has not changed since then - which means that Russia would likely suffer a humiliation in a direct military confrontation with the US - just as it initially did in confrontation with Germany in 1941. Unlike in 1941 however - when control of a territory was at stake - today the objective of a military confrontation would be limited to a show of force on the battlefield - as it was in Yugoslavia. Basically showing that NATO or US can bomb them and there is not much they can do about it.
The Russians are likely aware of that, which means that they will avoid direct military confrontation. It is an equivalent of remaining silent in response to criticism. People may think that you are stupid, but if you open your mouth they will know that for sure. Ditto for Russia's military response. If the Russians fail to respond to US aggression, others may think they are weak, but if they do, everyone will know that for sure.
-- Wojtek
"An anarchist is a neoliberal without money."