Wanna guess what happens after the US retaliates, then Russia retaliates and it goes down the spiral of "world's glowing green now"?

Again, nobody is stupid enough to let it get to that point. Nobody neither side wants to lose, and there is absolutely no winning in a MAD scenario.

Neither country will do anything to provoke the other to the point of retaliating against targets on their enemy's soil. Syria may become a flashpoint where Russia and the US fight each other in a Korean War Chinese sort of way, but that's about it.

So Russia could also walk over Europe and the US would be largely impotent to do anything, because fear of nukes and all. Right?

Honestly, it depends what parts of Europe Russia invades. We've already seen the US's impotence to resolve the Ukraine issue, and there's no way America is going to go to war with Russia over parts of Ukraine. If the Russians got ideas about Poland and Germany, well, then you might see some real fireworks.

But to be completely frank, I think this conversation is a moot point anyways. I this is all manufactured geopolitical drama, with neither side actually committed to fighting each other over little ol' Syria. Considering the airstrikes seem to leave all the important bits of the airbase intact, I can't believe this was anything but theater.