We can’t say we weren’t warned, and sometimes the warning comes from far away and goes unheeded. The boldly premonitory mockumentary “C.S.A.: The Confederate States of America,” which premièred in 2004 and was released in 2006, is coming back to IFC Center this weekend; it’s an ingenious vision of political, moral, and historical horrors. The premise is that the South won the Civil War; that the Confederacy annexed the Union and renamed the country the C.S.A.; that it turned out to be a pseudo-democratic, repressively racist state; and that this movie is a broadcast (complete with mock commercials and public-service announcements) of a long-suppressed documentary about the country’s history, made by the British Broadcasting Service.

The writer and director of this ferociously imagined and deftly realized film is Kevin Willmott, who, more recently, wrote the great “Chi-Raq” with Spike Lee (who is credited as “presenting” “C.S.A.”). Like “Chi-Raq,” the movie is an impassioned fantasy that reveals unbearable and nearly inexpressible realities—or, rather, blatantly oppressive realities that are so commonplace as to escape the notice of more or less everyone but their victims. Within its fictional framework, “C.S.A.” X-rays the depth and extent of American racism and the appalling ordinariness of white supremacy.

The show starts with an innocuous-looking faux commercial for an insurance company, with a white family frolicking on its suburban lawn to the voice-over of its cheerful young paterfamilias; the company’s slogan is “protecting a people and its property.” The “people,” of course, turns out to be a German-style Volk and the American white race (the word “Aryan” turns up frequently throughout the film), and the main “property” that Americans own turns out to be, yes, slaves, black slaves. “C.S.A.” tells a counter-story of how slavery survives to the present day—and it uses traces of contemporary pop culture to show that the notion isn’t really even such a stretch.

The “documentary” that’s broadcast features two main talking heads, a white historian named Sherman Hoyle (played by Rupert Pate) and a black historian at the University of Montreal named Patricia Johnson (Evamarii Johnson)—and even the detail of Patricia’s university setting turns out to be a diabolical giveaway. They tell the story of how, with the help of Great Britain and France, the South won the war, overran the North, burned New York and Boston to ruins, and reëstablished slavery in the period called Reconstruction. One of Willmott’s wildest inventions is his counterfactual story of Abraham Lincoln, who, to avoid capture by the victorious Confederacy, fled under the protection of Harriet Tubman, who had the idea to conceal him in blackface. Both were ultimately captured: Lincoln was tried as a war criminal, briefly imprisoned, and then exiled to Canada, where he died in the early twentieth century; Tubman was executed. Willmott includes both a filmed and recorded 1905 interview with the aged, regretful Lincoln, “remembered only as the man who lost the War of Northern Aggression,” as well as a clip from a faux film by D. W. Griffith—a victorious substitute for “The Birth of a Nation,” titled “The Capture of Dishonest Abe,” adapted from Thomas Dixon’s novel “The Yankee.”

Not only does the Confederate States of America restore slavery to the South, it extends it throughout the country, depicting the return of freed slaves to former owners and presenting the renewed slave trade as a cornerstone of the nation’s economy. Northern whites are induced to purchase slaves through tax credits that render the option very favorable, and Chinese immigrants in California are enslaved as well. Slaves seeking freedom, as well as abolitionists and people of principle—Mark Twain, Henry David Thoreau—flee north to Canada, where Susan B. Anthony leads a successful drive for women’s suffrage. In the modern-day Confederacy, women still can’t vote. What’s more, the C.S.A.’s policy of white supremacy is extended to Christian supremacy as well—no other religion is tolerated.

The film’s history of the C.S.A. involves wars of empire to control Central and South America, alliance with Nazi Germany (which is encouraged by the C.S.A. not to exterminate the Jews of Europe but to enslave them), and, on December 7, 1941, a surprise Confederate attack on Japan. The C.S.A. builds a wall along the length of the Canadian border—the “Cotton Curtain”—to keep slaves from fleeing. Footage of a 1960 Presidential debate between John F. Kennedy and Richard Nixon ingeniously repurposes Kennedy’s remarks about a world “half free, half slave” to refer not to Communism but to the Confederate institution, and to depict Kennedy as a covert abolitionist whose assassination prevented him from realizing his intentions. The mid-sixties riots in Watts and Newark are depicted as “slave rebellions”; protests and revolts in the nineteen-seventies were brutally suppressed, and the Reagan Administration restored “family values” that overcame such “self-doubt” about C.S.A. racism.

Willmott’s brilliantly imaginative fictions are on especially nauseating display in the film’s many faux commercials. The broadcast features an ad for “the Shackle,” an electronic device to clamp on slaves and track their movements, another offering special pacifying medication to maintain control of them, and yet another for separate and grotesquely unequal medical services providing “freedom medicine” to treat “Negro peculiarities.” The Slave Shopping Network is a home-shopping channel for slave trading, and a logo in the corner of the screen exhorts viewers to “use MassaCard.” Other faux ads feature products, such as motor oil, a restaurant chain, and a brand of tobacco, that bear unspeakably offensive racist names, and another promotes a sitcom with a bubbling Mammy-like maid in a suburban home, “Make Room for Beulah.” I’ll stop here, and add only that, while the details of these commercials are Willmott’s invention, their names are in fact authentic: a postscript in the credits documents their real-life history, which in some cases extends until late in the twentieth century. “Beulah” was an actual TV show; footage of a faux program called “Runaway,” in which Confederate Bureau of Investigation agents capture escaped slaves, appears to be taken from actual news footage of the arrests of black Americans.

Above all, the faux commercials capture the blithe contentment of white people, a fantasy of white supremacy that is inextricable from American history—and continues to sell goods and win elections today. The crucial thread of “C.S.A.: The Confederate States of America” is the delicate balance on which even the current impaired state of civil rights depends, the ingrained premise of racism on which the United States is built, the relentless effort that it takes to resist the realization of that premise and the restoration of its institutions, and the tiny flicks of historical switches on which matters of the gravest moment depend. Return to the beginning of Willmott’s film: he depicts a Confederacy that triumphed only owing to a dubious alliance with America’s enemies for the sole purpose of maintaining white supremacy.