Speaking of American exceptionalism, Walter McDougall reviews the history of the idea and identifies the origins of its current usage in the early Cold War period:

Exceptionalism dovetailed perfectly with a new orthodoxy among political scientists that extolled what Harvard professor Louis Hartz called America’s Liberal Tradition. Born free of an aristocracy and national church, Americans had no need for Europe’s ideological radicalism or, for that matter, its anti-ideological conservatism. Historians such as Daniel Boorstin, soon to be Librarian of Congress, traced American exceptionalism back to Plymouth Rock and made it the principal trope of a generation of textbooks. Sociologists such as Seymour Martin Lipset put the concept to good use as a heuristic device. Most of all, the idea of an America set apart by Providence and endowed with a special mission to reform (not to say redeem) the whole human race dovetailed perfectly with the political rhetoric needed to rally Americans to lead the Free World in what amounted to a holy war against “godless Communism.”

The Cold War origins of the current prevailing use of the phrase make it curious that it has become such a popular and overused phrase in the decades since the dissolution of the USSR and the collapse of communism in Europe. McDougall notes this and offers and explanation for why this might be:

If the term was of Cold War vintage, why do computerized word-searches show that references to American exceptionalism exploded—literally from hundreds to tens of thousands—only after the Cold War was over? My historian’s instinct tells me the question itself is the answer: The Cold War was over, globalization and multiculturalism were the new trends, and American identity was disputed as never before. What made exceptionalist rhetoric ubiquitous was the fact it was now contested and therefore deployed by almost all sides in the cultural wars of the 1990s, foreign wars of the 2000s, and political wars of the 2010s.

In sum, the myth of American Exceptionalism, ironically inspired by Roman Catholics and Marxists, entered our lexicon as historical gloss for the campaign to persuade a skeptical, war-weary people that global commitments such as the UN, the Truman Doctrine and NATO were not really a break with tradition but a fulfillment of the nation’s hoariest, holiest calling. Exceptionalism was not an archetype of the Promised Land but an artifact of the Crusader State.

So it is no surprise that the current usage has enjoyed the strongest revival among those hawks most eager to overrule and ignore a war-weary American public once again by continuing to link it with their preferred hyperactivist foreign policy.