A modernized China could easily enter a CW with the USA, as could a stronger Germany, an Alt-Russia, a unified and focused India, etc...
Really, all you need is another nation to control enough land, population, and resources to be able to go one-to-one against the USA. As for the destruction of other major powers that could interfere, there are many methods to achieve such. They could collapse into anarchy and civil war (China, Spain), they could have been ruined in war (Britain, France, Germany, Japan), they could've stagnated and collapsed (USSR).