What will happen with America in WW3?

China has become a Superpower in the World. The Russians are not changing their military either. The U.S. Is still the worlds most powerful nation but as all powerful empires come to an end i.e. The Greeks,the romans,the vikings,the Nazis,etc. So Some country will come and challenge the US. If America is screwed then What will become of the land? Leave your thoughts in the answers section.