In his State of the Union address this year US president Barack Obama said "We are the first nation to be founded for the sake of an idea." This is a staple theme of what is called "American exceptionalism".
The unique idea that America was founded on was that of God-given rights and liberty. The belief expressed in the Declaration of Independence is that "all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness."
So was born a country without an aristocracy, free from the feudal horrors of Europe. Foreign observers like French writer Alexis de Tocqueville were struck by the freedom and equality enjoyed by early Americans.
This allowed for a vigorous democracy quite unlike anything else at that time. Hence American politicians often use phrases like "the last, best hope of mankind'' or "the shining city on the hill''.
The American economy is a marvel in its dynamism and ability to raise the standard of living of huge numbers of people. It is a country of immigrants of great diversity who have integrated and become Americans.
Contrast this with countries whose nationhood is based on shared tribal, ethnic or language ties. This was not a possible basis for nationhood in South Africa with its many divisions and bitter history of internal strife.