When the Confederate States of America seceded, the response of the United States of America was firm: dissolving the Union was impermissible. By contrast, it took a few more years for the United States to resolve the question of whether it would permit slavery within its own borders, and it took more than a century for the U.S. to enforce civil rights and voting rights for all its citizens. This was mainly because of the South’s political power. In order to become the richest and most powerful country in the world, the United States had to include the South, and its inclusion has always come at a price. The Constitution (with its three-fifths compromise and others) awkwardly registered the contradiction between its democratic rhetoric and the foundational presence of slavery in the thirteen original states. The 1803 Louisiana Purchase—by which the U.S. acquired more slaveholding territory in the name of national expansion—set off the dynamic that led to the Civil War. The United States has declined every opportunity to let the South go its own way; in return, the South has effectively awarded itself a big say in the nation’s affairs.

The South was the country’s aberrant region—wayward, backward, benighted—but it was at last going to join properly in the national project: that was the liberal rhetoric that accompanied the civil-rights movement. It was also the rhetoric that accompanied Reconstruction, which was premised on full citizenship for the former slaves. Within a decade, the South had raised the price of enforcement so high that the country threw in the towel and allowed the region to maintain a separate system of racial segregation and subjugation. For almost a century, the country wound up granting the conquered South very generous terms.

The civil-rights revolution, too, can be thought of as a bargain, not simply a victory: the nation has become Southernized just as much as the South has become nationalized. Political conservatism, the traditional creed of the white South, went from being presumed dead in 1964 to being a powerful force in national politics. During the past half century, the country has had more Presidents from the former Confederacy than from the former Union. Racial prejudice and conflict have been understood as American, not Southern, problems.

Even before the Civil War, the slave South and the free North weren’t so unconnected. A recent run of important historical studies have set themselves against the view of the antebellum South as a place apart, self-destructively devoted to its peculiar institution. Instead, they show, the South was essential to the development of global capitalism, and the rest of the country (along with much of the world) was deeply implicated in Southern slavery. Slavery was what made the United States an economic power. It also served as a malign innovation lab for influential new techniques in finance, management, and technology. England abolished slavery in its colonies in 1833, but then became the biggest purchaser of the slave South’s main crop, cotton. The mills of Manchester and Liverpool were built to turn Southern cotton into clothing, which meant that slavery was essential to the industrial revolution. Sven Beckert, in “Empire of Cotton,” argues that the Civil War, by interrupting the flow of cotton from the South, fuelled global colonialism, because Europe needed to find other places to supply its cotton. Craig Steven Wilder, in “Ebony & Ivy,” attributes a good measure of the rise of the great American universities to slavery. Walter Johnson, in “River of Dark Dreams,” is so strongly inclined not to see slavery as simply a regional system that he tends to put “the South” in quotes.

After slavery had ended and Reconstruction gave way to the Jim Crow system, the Democratic Party was for decades an unlikely marriage of the white South (the black South effectively couldn’t vote) and blue-collar workers in the North. This meant that American liberalism had a lot of the South in it. Ira Katznelson, in “Fear Itself,” adeptly identifies the deep Southern influence on the New Deal era, the country’s liberal heyday, including not just its failure to challenge segregation but also a strong pro-military disposition that helped shape the Cold War. The great black migration to the North and the West, which peaked in the nineteen-forties and fifties, partly nationalized at least one race’s version of Southern culture, and, by converting non-voters to voters through relocation, helped generate the political will that led to the civil-rights legislation of the nineteen-sixties. Once those laws had passed, the South became for the Republican Party what it had previously been for the Democratic Party, the essential core of a national coalition. The South is all over this year’s Republican Presidential race.

I’m a fifth-generation Southerner, though long expatriated, and I know the wounded indignation with which the folks back home react to any suggestion that the South is no longer—or maybe never was—an entirely separate region. What about our hound dogs, our verandas, our charm, our football worship, our slow-moving “way of life”? Outsiders who have visited the South, going back to Alexis de Tocqueville and Frederick Law Olmsted or even further, have usually agreed with the natives about the South’s distinctiveness, though they have often seen it as something to condemn, not admire. How can the South be so American if it feels (and smells, and sounds, and looks) so Southern?

One of the many categories of visitors to the South was concerned liberals during the New Deal, who were primarily interested not in race but in “conditions”—poverty, disease, ignorance. These included the documentary photographers dispatched by the federal government’s Farm Security Administration, who wound up creating most of the familiar images of the Depression, as well as anthropologists, sociologists, journalists, social reformers, artists, and filmmakers. James Agee and Walker Evans’s lugubrious book “Let Us Now Praise Famous Men” is one of the most enduring examples of this tradition. (The 1941 Preston Sturges film “Sullivan’s Travels” manages the nearly impossible feat of poking fun at such visitors while also making it clear that their mission had a powerful moral justification.) During the same period, white Southern novelists produced their own body of work that trafficked in Southern dispossession and dysfunction. William Faulkner was at the head of this class, which also included Erskine Caldwell (who was part of the social-documentary tradition, too, through his professional and personal partnership with Margaret Bourke-White) and, later, Carson McCullers and Flannery O’Connor.

Paul Theroux, the veteran travel writer, seems to have prepared for “Deep South: Four Seasons on Back Roads” (Houghton Mifflin Harcourt), the first of his ten travel books set in the United States, by immersing himself in these works from the second quarter of the twentieth century. The genre in which he is working naturally organizes itself into vignettes rendered with a primary focus on literary artistry, rather than analysis, so he never has to state a full-dress argument, or even say exactly what he was looking for in those four long driving tours. The South remains more rural than the Northeast, but by now, as in the rest of the country, most people live in metropolitan areas. Still, Theroux tells us, “I stayed away from the big cities and the coastal communities. I kept to the Lowcountry, the Black Belt, the Delta, the backwoods, the flyspeck towns.” This principle may have been a way of simplifying his writing assignment: these are places where some people eat squirrels and raccoons, and are obviously unusual in a way that people in the Atlanta suburbs are not. That makes them easier to portray vividly. But Theroux is left trying to evoke the fastest-growing region of the country, where a hundred and twenty million people live, by taking us to a series of poor, deep-rural, depopulated places, like Hale County, Alabama; the Mississippi Delta; and the Ozarks, where the main noticeable changes in the past few decades are outsourcing and the advent of Gujarati Indians as motel owners.