Let them, let's see what they will do. I wonder who would win such a conventional war. Though I guess afterwards there would be no Hong Kong left protecting.
Certainly both would lose many lives. Why would any American want to die for this crisis? Why would you imagine China would give up or be unable to defend territory so near their mainland?
In a defensive war, probably China. They've got more tanks, more people, more manufacturing capabilities, lots of artillery and no doubt they've spent years preparing themselves militarily for such an event.
China has enough nukes for this to be a moot point. The outcome would be the same as if China decided to annex Hawaii -- an all out war with nuclear and other WMD.