Who said anything about invasion. Stop thinking in those terms, expand your scope.

What makes the US a superpower?

It's not military might.

Sure, we can threaten death to anyone we want. But, what if the world "sanctions" us Charles?

We cannot say, keep shipping us oil. Keep shipping us coffee, tea, sugar, etc. etc. We cannot FORCE the world to keep doing business with us if they decide to stop.

Particularly since they can rally around China now.

The decisive moment will be when China sanctions the US. If enough of the world follows them. It's over for us. Without a shot being fired.

The greatest victory is that which is achieved without battle. -Sun Tzu