China is a tough one. What they have is people. A LOT of them. In an infantry war, there would be little contest. We lose, plain and simple. What we do (currently) have over them is technology. We have many more planes, ships, subs, etc that do a lot to makeup for sheer numbers of available soldiers.
There are a few factors to consider though:
1) China, FOR THE MOST PART, has not really been that interested in expansion. They have for much of history been INCREDIBLY isolationist in nature. They're more about holding onto their land than getting additional land. The big (*) next to that is that they've also been very adamant about claiming (or reclaiming) land that they consider to be part of China. This currently includes Taiwan, a current US protectorate and ally.
2) China is heavily entangled with the US economically. We own them a TON of money, and if we were annihilated those debts would never get payed. If the US were gone tommorow, it would have devastating consequences on the Chinese economy.
3) When any two large nations like that go to war, it's going to be devastating for both sides, regardless of which side wins. That's part of what kept the US and the USSR away from war for so long: the sad truth is a war would have spelt the end of both of us, regardless of who ended up surrendering to who.
With that in mind, with the slight wildcard being Taiwan, I just don't see us having military differences with China (outside of your standard cold war style escalation). It'll moreso be a matter of economic competition, and in that arena, China is beating us silly, as are others. Our manufacturing jobs are going to China whilst our white-collar jobs are going to India. There's a lot of talk about us transitioning to a "service based economy", but that will never work - other countries will not keep sending us goods for us to sit around "providing services" to one another all day. Without SOME manufacturing sector we'll just keep sinking lower. Sad reality is that that sink won't stop for a LONG time. Chinese/Indian standard of living goes up, ours goes down. Our population density is much lower, so we have a natural advantage now, but the sheer economics of it won't settle out until the two sides come close to balancing. IE, until Chinese and American labor becomes similar in price.
We're in for a rough ride.