The idea of "American exceptionalism" claims that, compared to other nations, the United States of America is "exceptional"—unique, unusual, or special, and not to be held to standards of other nations, but rather a completely unique political or social state with a special historical mission (whose description may vary).

Crude versions of this idea assert that God loves Americans better than other people because Americans are better than other people—just fjor being Americans. See also variations developed by American neoconservatism below.

Informed discussions of American Exceptionalism highlight two important, related aspects of American national identity:

1. The USA's status as "a nation of immigrants" means that the American nation is defined not by a particular ethnic or national group but instead of many different peoples who blend together to form a new, modern identity defined not by ethnicity or background but by beliefs, universal ideals, and some governing cultural traits.

This aspect of American Exceptionalism rises from the premise that most nations are comprised by a single ethnic group who speak a common language: England is full of Anglo-Saxons who speak English; France of French people speaking French, etc.

Crevecoeur's Letters of an American Farmer (1782) is a foundational text of American Exceptionalism in that it establishes that Americans cannot be identified ethnically—they are not English, French, German, but a mixture or melting pot.

Comparably, immigrants speak many languages before learning English, but learning English doesn't make them English. The English language is mostly a matter of simplicity and convenience, and American language itself becomes a somewhat new and different specimen of English featuring contributions from African Americans, immigrant Americans, Spanglish, etc.

2. American identity or culture is defined not by race or ethnicity but by beliefs.

But if the USA is a nation and nations are usually unified by ethnic or cultural homogeneity, how is the USA unified as a single nation?

In place of ethnic identity, Americans are identified by a set of beliefs or attitudes—"Americanism" maybe, but also liberty, human rights, freemarket capitalism, limited government, guns, mobility, progress (or decline), populism (what the people want rather than what elites or experts decide). 

The set of beliefs may also be characterized as "the American Dream" and may be prefigured by the USA's population primarily by immigrants and their descendants, who are predisposed toward the American Dream of socio-economic progress and preservation of individualism and family values.

The nation's Founding documents—The Declaration of Independence and The U.S. Constitution—express these rights and values while also implying groups or identities whose inclusion in such citizenship is not automatic. 

In recent decades "exceptionalism" has been associated with American neoconservatism, esp. during the 2nd Bush-Cheney Iraq War:

• rejection of international treaties

• dismissal of the United Nations and its weapons inspectors

• use of torture despite international restrictions

• no knowledge of Iraq or its people (e.g. that all Muslims are not the same) but instead an assumption that they want to be just like Americans

• no budgeting or taxes to pay for war—instead, tax cuts for the rich!

All this and more were defended as reflecting America's exceptional power to do whatever it wanted regardless of international opinion or common sense and decency. In the words of one of the then-president's advisors, "we create our own reality."

When President Obama attempts to work more respectfully with other nations and international organization, such cooperation is seen as evidence he doesn't believe in American Exceptionalism and therefore isn't truly American but a Muslim communist from Africa. 

http://www.gseis.ucla.edu/courses/ed253a/american-exceptionalism.htm

http://www.hutchnews.com/Columns/Price-column-4-06--1