USA of course because NATO would get in involved. The EU would be fighting the Russians on the European front while since the USA is at war with China, China loses it's number one reliable trading partner. China is already economically destroyed while all the USA has to do is block the coast and China is pretty much on life alert. We use our superior air force to bomb the internal infrastructure of China to make things worse. China's forces are initially spread out because they are probably fighting two fronts in Korea and South East Asia. We slowly push back their invading forces until we've reached Han China, the heart and we just kick in through there. No need to invade far western China, Manchuria, or Inner Mongolia because they're mainly buffer states. Once China is out, the EU forces are probably fighting in European Russia right now, so the USA invades through Siberia and bam. Also, lol North Korea.
Or the USA could lose because we lose 200 troops and the home front is already wanting to end the war lol.