China simply can't invade the US. Sure they could call up close to 200 million troops but how would they get here? Our Navy would blow them out of the water before they even got out of port.
OTOH, if we invaded them China would just nuke our troops as they landed or overwhelm us with vastly superior numbers. On neutral ground, we'd utterly destroy them, now, but in 20-30 years this may not be true. In fact if China opened up and became buddies with everyone they could quickly become the most powerful nation in the world because they could get technology easier.
I don't believe our reign as the big boy on the block is over just yet but I don't think there's any doubt that we're declining and China is rising. It's only a matter of time before they overtake us. Only us destroying them now can prevent this but if we started crap with China there's a good chance it would be TEOTWAWKI when the nukes start flying. Especially if Russia got involved.
There are limits to china's current growth. Anyone who has taken the most basic level economics class in high school can see this. There are a lot of problems with china's current economic system. Ridiculous amounts of unemployment, growing disparities in income levels, a health system that is in shambles, waning energy sources, an unstable financial system, corruption, and environmental deterioration.
These same things are happening here too.