Forebodings.

At Canaan's Edge: America in the King Years, 196-68, by Taylor Branch (Simon & Schuster, 1,056 pp., $35)

FORMALLY, At Canaan's Edge is the third and final entry in Taylor Branch's magisterial chronicle of the civil-rights movement, covering 1965 to 1968. The book is especially valuable, however, in explaining later events--in showing how we got from "I Have a Dream" to "Kill Whitey."

Opening with "Bloody Selma" in 1965, Branch goes on to cover a vast array of developments that we can barely believe occurred within just a thousand days. The year 1965 alone witnessed not only Selma, but the Voting Rights Act, the Immigration Reform Act, the Moynihan Report on the roots of black poverty, and--more ominously-the torching of Watts by its own residents. Meanwhile, the impassioned young nonviolence disciples of the Student Non-violent Coordinating Committee (SNCC) were slowly but steadily teaching poor blacks in Mississippi and Alabama how to vote and form parties. After the Birmingham and Selma victories, Dr. Martin Luther King Jr. tried to bring the protest-march model to Chicago's slums, only to meet searing racist resistance from white crowds ("I have never in my life seen such hate") and negligible concessions from Mayor Daley.

Afterward, King changed his focus to poverty in general, planning a race-neutral sequel to the March on Washington--a project about which few of his supporters were enthusiastic. King was actually floundering somewhat at this point, unsure where to go next now that the concrete victories of the Civil Rights and Voting Rights acts were accomplished. A strike among black sanitation workers in Memphis attracted his attention while he was at this loose end, which is how he came to be in that city when he was assassinated.

Branch also provides detailed coverage of J. Edgar Hoover's obsessive eavesdropping on King's philanderings, and of his endless quest to prove that King was backed by Communists. Hoover and his men at the FBI were so diligent in this that much of Branch's day-by-day coverage of King's activities is drawn from FBI reports.

The Bureau's pursuit of King is now, of course, old news; At Canaan's Edge is more useful in reminding us that when he was alive, contempt for King was by no means limited to southern bigots and twisted cases like Hoover. Even in mainstream opinion, King was regularly dismissed as callow and opportunistic. In this vein, Lawrence Spivak, interviewing King on Meet the Press, could openly tar the marches as excessive. Plenty of people who would today qualify as blue-state Americans classified King as wanting "too much too soon."

By the late Sixties, however, his call for nonviolence--which, after all, meant young black people allowing whites to beat the daylights out of them--was being widely rejected as an unrealistic expectation of a people who had been through so much already. None other than The New York Review of Books pronounced that "whites have ceased to believe him, or really to care; the blacks hardly listen." The Watts rioters greeted King's attempt at intervention with "Get out of here, Dr. King! We don't want you."

This last sentiment is the essence of Branch's narrative. In the wake of Watts, President Johnson asked 500 civil-fights leaders a question that interests curiously few historians: Why did Watts bum the same week that the Voting Rights Act was passed? The problem was a new type of black "leader" for whom, as Joseph Alsop put it, "injustice is the theme, not what can be done about it." King was well aware of the threat from this contingent, criticizing a new kind of SNCC demonstration as "expressions of rivalry and rage, without constructive purpose."

Exhibit A was Stokely Carmichael, who began as a diligent SNCC worker gaining poor blacks the vote in the Deep South. In 1965, he condemned "floaters" in the organization who were devoted to drama over activism. But the following year he announced that he had rejected nonviolence as unrealistic when looking down the barrel of a gun. Hardly an unreasonable proposition--but rejecting nonviolence does not require abandoning being constructive. This is, however, precisely what it meant for Carmichael and his ilk: From then on, all we got from him was charismatic but empty apergus (such as "Integration is a subterfuge for white supremacy") and calls for a "Black Power" devoid of any political program. Starting in 1966, Carmichael was the head of SNCC, and by the time King was killed, the Black Panthers were outperforming even Carmichael--calling for armed riot to the delight of a certain contingent, but never putting their money where their mouth was.

These elements did succeed in spurring a new culture. In the Memphis strike, the old guard's placards said I AMA MAN and MACE WON'T STOP THE TRUTH. But on the sidelines, teenagers imprinted by a seductive new gospel held signs saying BLACK POWER IS HERE and, to Memphis's mayor, LOEB EAT SH**--which left open the question what Loeb was to do after performing that unlikely act, given that he would still be in power. A riot followed that was the last thing King wanted, and he was killed seven days later. After this, the likes of "Loeb eat sh**" placard-bearers led directly to Al Sharpton's embrace of Tawana Brawley, rapper Kanye West's "George Bush doesn't care about black people," and other gestures by black figures who think they channel King by merely playing the rebel for the cameras.

The publication of At Canaan's Edge also sheds light on the ways in which 2006 is better than the Sixties. Today, a comprehensive chronicle of the civil-rights revolution gets top-class media coverage and sells widely. In 1965, when Los Angeles's mayor and police chief could casually proclaim after Watts that they knew of no racism in the city, a civil-rights chronicle would have had a hard time interesting a commercial publisher.

But there is a double-edged sword here, in that thinking people's culture has so thoroughly internalized the concept that "race matters" that the sheer existence of Branch's trilogy figures larger in most critics' estimation than its quality of composition. At Canaan's Edge, while usefully getting a massive volume of facts between two covers, is not precisely history written with lightning, and few will be inclined to read it all the way through. Branch reports more than he interprets, and his narrative crackles only occasionally. In the first two books this was less of a problem, because civil-rights history until 1965 was dominated by clean triumphs with beginnings, middles, and ends. But after this came fragmentation and questions; as a result, At Canaan's Edge is exhaustingly episodic.

Beyond the famous figures, we meet a confounding number of functionaries in the movement, the government, and the FBI--people who are so interchangeable that it becomes difficult to keep up with the cast of characters (and Branch's editor has addressed this problem only slightly). Branch also devotes too much attention to Lyndon Johnson's grapplings with Vietnam. King's opposition to that war is obviously germane to his overall biography, but detailed accounts of how the war progressed and Johnson's responses to the same have been too well covered elsewhere, and here just make the book a good 200 pages longer than it need be.

At Canaan's Edge remains valuable as a fine-grained account of how and why a sober movement that brought America closer to its democratic roots left behind the self-medicating kabuki of identity politics. When many of black America's brightest are pretending that rappers' nihilism is uplifting, it would appear that too many are losing sight of the difference between activism and acting up. Branch's text brings that difference back into sharp focus.

Mr. McWhorter is a senior fellow at the Manhattan Institute, and the author most recently of Winning the Race: Beyond the Crisis in Black America.