Apart from a stalemate situation in which both sides don;t think it is worth it going all the way, I think China will lose if it was a bitter - no man left alive - situation like the WW2 with Japan-US.
Can you think of US losing the war? Probably not. So the only other alternative is China losing , whatever the cost.
It is hard to speculate on such a topic, but I think that it is fair to say that China could indeed be defeated in a major war. While they do have a large number of citizens, the advanced weapons that the USA has could go a long way towards us winning.
China has a huge army and a huge population from which to get reservists or replacements for their army. They could undoubtedly fight a long, tough war. They would most likely win against most countries. However if they went up against an ally of Japan, the United States, or NATO, it is likely that China would lose after awhile.
China is a world superpower. And they have enough people to the extent where they would probably outpopulate you westerners. Also, China has enough technology to probably hack most of the attackers, then bomb the rest, and march in and claim the countries. Lastly, China has several allies, like North Korea, who would send nukes and sink islands. And several countries depend on China's manufacturing, and the labor force would have no problem focusing on the nation's military instead of global merchandise.
Why are we even thinking about this? I do not believe that they could be defeated in a major war, 1) they have a ton of people, and 2) we are SORT OF in debt to them, and I would hate to think what would happen if we got on their bad side. If it was US vs. China, I believe that China would win, but I don't get why we are even thinking about this. We have no quarrel with China, and we should not create a quarrel with China.
Why are we even considering another war? China is no major threat, we still buy from them, and even if they are not peaceful, another war would be pointless. Many lives will be lost, families torn apart, children on both sides orphaned, and for what? power? War does not solve anything but terror and death.