A question from Yahoo! Answers:

What was it that caused/allowed the United States to become the economic superpower of the world?

The U.S. GDP per capita is not exceptionally high by the standards of developed world; it is, therefore, the sheer population factor that makes the U.S. economy so large compared to the rest of the world.

Before World War I, the U.S. population was about 50% greater than Germany’s; now, it is 250% greater. The U.S. population simply grew much faster than German population. So the answer to your question is rather obvious: immigration and relatively high family fertility…