In the end, of course, the Soviet and Japanese threats to American supremacy proved chimerical. So Americans can be forgiven if they greet talk of a new challenge from China as just another case of the boy who cried wolf. But a frequently overlooked fact about that fable is that the boy was eventually proved right. The wolf did arrive -- and China is the wolf.

The Chinese challenge to the United States is more serious for both economic and demographic reasons. The Soviet Union collapsed because its economic system was highly inefficient, a fatal flaw that was disguised for a long time because the USSR never attempted to compete on world markets. China, by contrast, has proved its economic prowess on the global stage. Its economy has been growing at 9 to 10 percent a year, on average, for roughly three decades. It is now the world's leading exporter and its biggest manufacturer, and it is sitting on more than $2.5 trillion of foreign reserves. Chinese goods compete all over the world. This is no Soviet-style economic basket case.

Japan, of course, also experienced many years of rapid economic growth and is still an export powerhouse. But it was never a plausible candidate to be No. 1. The Japanese population is less than half that of the United States, which means that the average Japanese person would have to be more than twice as rich as the average American before Japan's economy surpassed America's. That was never going to happen. By contrast, China's population is more than four times that of the United States. The famous projection by Goldman Sachs that China's economy will be bigger than that of the United States by 2027 was made before the 2008 economic crash. At the current pace, China could be No. 1 well before then.