Hm... well, how is very important to finding out what happens...
Now, China isn't as ASBish as The Red make it out to be- China disliked the German and Habsburg concessions, yes, but they disliked the French, British and Russian concessions just as much, if not more, and they did join the Entente in OTL. Given China's situation in 1914, and even more so later in the war, having them enter war against anyone that is in good position to strike against them does not seem likely at all, however- I pulled it off in A Central East, but that required things like a neutral Britain and an Allied ('Central Powers') Japan, and it still didn't happen until Russia was out of the game. But if you have China being more politically united during the Great War, with the following effects (something like Hendryk's Superpower Empire TL, but perhaps even more) that brings, it might be possible to get away with less.
The USA is harder- isolationism was deeply ingrained, so you'd basically have to have one of the Entente powers attack them (or 'Briand telegram' them) to get them to join. Or, perhaps, escalate tensions gradually, but that, again, most likely requires a pre-Great War POD.