How did the US become the most powerful country in only 200 years time?

It seems to me that China, which has been a country for thousands of years, would be eons ahead of every other country in the world. Or Great Britain for instance... they have been a world superpower since long before the middle ages. How did they lose their grip? America has only been a recognized country for a little over 230 years. We started from scratch, and look at us... pretty much running the world. What was the big "a-ha" moment that took us from being just a fledgling country to being the big man on campus? Was it our entrance into World War Two? That's what I think, but what are your thoughts?