The year 1968 is synonymous with political drama: The violence at the Democratic National Convention and the riots in more than 100 American cities, the assassinations of Robert Kennedy and the Rev. Martin Luther King Jr., the near-revolution in France, the Prague Spring, Khe Sanh and the Tet Offensive, Black Power at the Olympics, Enoch Powell and his “Rivers of Blood,” the shooting of Andy Warhol by a radical feminist, The Troubles in Ireland, the massacre at Tlatelolco in Mexico, on and on.

And what came out of all that chaos 50 years ago? A landslide for Richard Nixon, who promised a Warren G. Harding-style “return to normalcy” after the anarchy of 1968.

And so began the Great Turn Inward.

Americans were exhausted — and disappointed — by the long run of political radicalism that ran in spurts from the New Deal through the Great Society, and it is not surprising that in 1968 they turned to man who had last held office as Dwight Eisenhower’s vice president. But the turn in 1968 was not toward conservative politics but away from politics per se. It was a turn away from res publica toward a soft and vulgar hedonism — a social, sexual and spiritual retreat enabled by new possibilities in entertainment and technology that also appeared on the scene in that very eventful year.

Consider the other events of 1968, each of them more consequential than a dozen Tet Offensives: Intel was founded, and the Pentagon approved development of ARPANET, which would become the foundation of the modern Internet. The last steam locomotive made its final voyage in the United Kingdom while Boeing rolled out the first 747 in the United States, announcing a new era of widely accessible air travel that would render the term “jet set” and its connotations of wealth and glamour quickly obsolete. Americans orbited the moon. The modern technological era began.

We should have guessed what would become of that new technology, because we had been there before: In the earliest days of television, techno-utopians predicted that the new technology would be used to allow workingmen to sit in on Harvard lectures and enjoy the Metropolitan Opera. The reality was Ed Sullivan and Howdy Doody. The personal computer and the Internet would become in no small part instruments of pornography, one of the most influential social forces of the 21st century. Porn began to go mainstream in 1968 with the release of Warhol’s “Blue Movie,” and only a few years later the era of Porno Chic would be in full flower, with “Deep Throat” playing in mainstream commercial theaters and Frank Sinatra and Barbara Walters in the audience. The star of that film appeared on William F. Buckley’s “Firing Line” to debate obscenity laws.

Polarization is only a symptom of the underlying absence of a common culture

Citizenship is difficult work. It is far easier to pursue private pleasures, be they sublime or profane. And as technology becomes more powerful and entertainment becomes more lifelike (“Planet of the Apes,” with its innovative prosthetic makeup, debuted in 1968) what began as diversion becomes immersion. Civic engagement declines as each of the various tribes and castes of this American life retreat, each into its own cocoon of parochial interest and shared cultural affinities. Citizenship itself becomes impossible because the would-be citizens hold so little in common with one another that those bonds of affection that so concerned Abraham Lincoln never form. And what the people do hold in common are only items of consumption: The Big Mac, the pornography of food, also made its debut in 1968.

We worry a great deal about the polarization of American politics in the 21st century, but that polarization is only a symptom of the underlying absence of a common culture, of permanent things shared. Citizenship is a shared thing, and it cannot be shared between hostile alien peoples who happen to be occupying the same territory. That, and not the top marginal tax rate, is what the ongoing national convulsion from George W. Bush to Barack Obama to Donald Trump is really about.

Citizenship also requires superseding one’s self, something that is rarely even attempted in our age of identity politics, in which every fact and development of community life is strained through the narcissistic filter of the most infantile kind of self-obsession. It is notable that the great political motto of our time is “Me too,” as though the world ended where our noses do.

We are a long way from Valley Forge and Gettysburg.

Or Apollo 11, for that matter. A month after “Planet of the Apes” was first shown, “2001: A Space Odyssey” continued the journey inward. Space exploration was no longer the frontiersmanship of “Star Trek,” but a cracked psychedelic trip that climaxes with the protagonist’s transformation from an adult scientist into a kind of galactic fetus floating amniotically in space and contemplating an Earth of which it may or may not be aware. That image represents the entire culture of 1968 and after in silent apostrophe.

The radicals of 1968 thought they were getting a revolution. What they got was involution, a generational retreat into childish self-absorption.

“Turn on, tune in, drop out.”