If any other country in the world simply attacked another sovereign nation because they felt like it then the repercussions would be very harsh indeed.
Hmm, I can think of several recent examples where there were no repercussions at all (North-South Korea, Russia-Georgia, US-Iraq).
It is intriguing all this fanfare about the S-300 given there is no air threat to the regime at present. Were the Saudis or Qataris, or maybe the Turks, up to something? Not sure if the former has any airbases in the north of their country - if so these would be no more than a few tens of minutes flying time from Syrian airspace.