Complete hypothetical: Putin bombs Poland and/or the Baltic states.
How would the world respond? What would NATO do? What would China do? Who would support Russia?
This would be full out war, kind of WWIII, but not the same scale as WWII, since Russia wouldn't have many allies. China would not ally with Russia.
I agree that China would likely sit out if a conflict between Russia and the U.S. broke out. In reality, Russia's only allies would be Iran, Venezuela, Cuba and North Korea. On the other hand, our main allies would be all the NATO member states, South Korea, Japan and Israel.
