That's quite incorrect, because there will be no US for Europe to help in your scenario, nor Russia to help against. Both would be irradiated no-go land.
This is something many US residents tend to forget because they didn't have a war against an aggressor capable of fighting on their land for centuries. To them, war is something that happens far away with no real impact on their lives. A war with Russia on the other hand would be a war against a foe capable of unleashing the real consequences of the war against the people of US.
And mind you, US is doing a lot against Assad. The real problem is that people in White House right now are not quite as "american cliche" in their utter ignorance of situation on the ground and understand that any help against Assad equals improving Iran's position in the region, one way or another. Just like Iraq intervention ended up being nothing more than surrendering Iraq, the former anti-Iran bastion straight to Iranian hands. As a result most of anti-Assad action taken is typically in the form that avoids straight up supporting Iran's position in the region, which severely limits potency of any measures taken. It's mainly undercover support of semi-independent Sunni militias and support for the only anti-Iranian force in the region - ethnic Kurds. And then there's of course the political pressure on allies of Allawites.