What major military power was allied with Saddam Hussein in Iraq when the U.S. invaded? None. What major military power was allied with the Taliban in Afghanistan or Gaddafi in Libya? Again, none. It's different this time. Russia and Iran are Syria's ally.
Play out the scenario. Russian surface-to-air missiles knock down some cruise missiles but not all of them. The bombs hit their intended Syrian targets on the ground. Some of the Russian military "advisers" giving assistance to the Syrian army are killed in a collateral damage strike near the various target groups.
Russia launches fighter aircraft on stand-by support in anticipation the U.S. coalition forces (U.S., Britain, France, possibly Israel) will do aerial reconnaissance overflights to verify target damage. Some of the Russian federation planes fly near the coalition aircraft to intimidate them into leaving immediately. Another group of federation fighters head out to the eastern Mediterranean Sea. The planes locate the U.S. naval destroyers and loiter in the area, perhaps buzzing over them to warn them to cease action.
Now events get dicey and extremely tense. All it takes is one or more of the pilots in their planes or naval personnel at their general quarter stations to misunderstand a superior's order or they panic and start engaging with weapons fire and you've got military casualties on both sides. Now it becomes an air sea battle. Which side will break off first and retreat? Which side will press on and intensify hostilities?
The media on both sides ramp up the war rhetoric to prosecute their handler's agenda. There are "diplomatic" exchanges thru official channels as well as backdoor communications. The military commanders in the field either obey their political leaders as directed or they go off the deep end and expand into a regional war encompassing all of the Middle East. These events will read like a Tom Clancy novel only they are real and deadly.
Obama must have an epiphany, stop acting like a puppet king and rein in these blood thirsty neocons or the world will plunge into an abyss of war and suffering that will take decades to recover from, if ever in our lifetime.