Q: "How did the United States become a rich country?"
A: "Free-market capitalism."
(e.g., http://www.amazon.com/Bottom-Billion-Poorest-Countries-Failing/dp/0195311450 and http://www.amazon.com/Lexus-Olive-Tree-Understanding-Globalization/dp/0385499345/ref=sr_1_1?s=books&ie=UTF8&qid=1329149909&sr=1-1)
Oops, they forgot to mention, the United States had the highest average tariff rates in the entire world from the Civil War until the 1930s.
https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEij_Bp_ivL6Mm8S7-NXJUv9CdDMIEPbUU-bgH5XNl-ZKy6JEusXlt4Kqyi6_Rmog3yj1OsrAhPDChKo_tBxIlJh0dmg3yMo4VUSXgp8Ir7_zBiQ6fMdjI90JrlivW5ogI3zViQpRzYoo8Tw/s1600/protection+pic.jpg (from http://www.amazon.com/Kicking-Away-Ladder-Development-Perspective/dp/1843310279/ref=sr_1_4?s=books&ie=UTF8&qid=1329150057&sr=1-4)
Protection of infant industry is also how Communist China is becoming a rich country and why countries with plenty of natural resources and government completely out of the way of business (Mongolia, Somalia) are not rich. Oh yeah, and there were also those imperialism and slave-trade things (God decided to put the natural resources of Britain, Denmark, Switzerland, France, etc., in other people's countries. The US is, of course, Britain's colonial descendant.).
Repeat it three times: the United States had the highest average tariff rates in the entire world from the Civil War until the 1930s. The United States had the highest average tariff rates in the entire world from the Civil War until the 1930s. The United States had the highest average tariff rates in the entire world from the Civil War until the 1930s.
The idea that the US developed due to free-market capitalism is a fantasy used for political purposes.