There is a reason why we historians organize our classes on the history of the United States from the colonial era to the Civil War, and then from the Civil War to the presidency of Barack Hussein Obama. Almost two decades after the fact, the 2008 election remains a watershed in American history.

Countless writers more eloquent than I have described how American society, up to that point, had been divided. My students find this hard to believe, raised as they were in a world where skin color is just another accidental, like the length of one's hair or one's sexual orientation. But in 2008 Obama represented the quintessential Other. While generations of immigrants — whose native tongue was Gaelic, or Italian, or Yiddish, or Spanish — had managed to transform themselves into Americans, the descendents of enslaved Africans had never realized this promise. In the year of Obama's birth, Freedom Riders were beaten, shot at and burned. In his childhood, fire hoses and police dogs were turned on the children and grandchildren of slaves.

But even more than the color of his skin, Obama's name was thought to be an insurmountable barrier. Remember that seven years before his election, extremists adhering to a victimized, perverted form of Islam had transformed passenger aircraft into improvised missiles, thus also transforming Islam itself into America's quintessential boogeyman. And yet, Obama was elected as its forty-fourth president.

Why did this happen? To give the simplest explanation, in the late twentieth century, American nativist sentiment, originally expressed in Know-Nothingism, xenophobia and closed borders, coalesced into a thousand subtle and overt gestures meant to widen the gap between Us and Them. Firearm ownership, opposition to immigration, opposition to sexual freedom and access to reproductive medicine — all of these were totems of a tribe that, pressured by external economic factors, feared change and sought to defend itself from an imagined enemy. Playing on these fears, the memetic engineers of the New Right sold a plan of government deregulation that, in turn, increased the economic pressure in a vicious closed cycle. Obama's good fortune was to be up for election at the very moment when this cycle could no longer sustain itself.

Electoral victory, however, does not always translate into successful governance. Historians have attributed Obama's success in his two terms as President to any number of factors — his reversing the ruinous economic policies of the New Right; his use of technology to transform a patrician, republican system of representative government into a responsive, flexible direct democracy; his ability to convince a country with a frontier mentality of the value of social welfare. But Obama's success was rather due to the way he embodied transformation. The ostracism and fear of blacks was the single greatest impediment to American progress. Obama, by the simple fact of his being, breached this seemingly impregnable mental stronghold, and demonstrated the truth of the motto e pluribus unum.

This principle, as it turns out, is the hallmark of the most enduring cultures. Rome built its empire not by totalitarian subjugation, but by extending citizenship and, eventually, rulership, to conquered peoples. The so-called "barbarians" that ended the Empire were not unlettered brutes who tore down a great civilization, but emigrants trying to rebuild a collapsed system. (A negative view of the so-called "Dark Ages" has persisted only because our recorded sources were, in many ways, the Know-Nothings of their day.) In the same way, China was able to conquer its conquerors, from the Mongols to the Manchurians, by extending the benefits of its civilization to all. We see the same in the Indus Valley and in the Caliphate of Baghdad. If we seek a modern counterexample, we need look no further than modern France, where the Sixth Republic is experiencing painful social upheaval and loss of cultural identity, and the art treasures of the Louvre are currently being dispersed through museums in the United Kingdom and North America.

That the United States remains the world's "superpower" is a testament to what used to be called the "American Dream." The frontier, contrary to what Frederick Jackson Turner wrote, never closed; it rather became the frontier within our hearts. Obama's election marked the moment in American history when a human being could be judged not for the color of his or her skin, but for the content of his or her character. Not coincidentally, it also marked the moment when the United States turned definitively from a fortress of self-interest to a peaceful emissary of freedom and human rights. These are the principles that the pax Americana has been built on, and an inheritance that we hope to keep as a legacy for our children.