History is full of smaller powers defeating larger ones - our own War of Independence from the British Empire is just one of them. I know that you are going to rebut with the something about the French Navy and the other conflicts the the British were fighting at the time. My counterpoint is that if there is a war between the US and China it is not likely that India, japan, Australia, the U.K. and other powers would sit it out. You have no idea of the animosity and hatred between Japan and China and India is not a fan of China either.
All depends... on exactly what sort the war was, where and about what?
Naked aggression by China against its neighbors would be met by unified coordinated opposition.
A Chinese peace-offensive in one area, plus hullabaloo elsewhere to hide aggression in a third area (i.e., Taiwan) might work, especially if our own leadership is under the thumb of zombies who can't think beyond where their own extra paychecks are coming from.