Freedom Road: The Pathfinders : THE PROMISED LAND: The Great Black Migration and How It Changed America <i> By Nicholas Lemann (Alfred A. Knopf: $24.95; 320 pp.) </i>

Surely the most consequential event in the history of America was the establishment, in the late 18th Century, of a system of ordered liberty. But the institution of slavery runs a close second, and on at least two occasions, its presence in a society purporting to protect liberty has produced violent conflict. In both cases--the Civil War and the civil-rights struggle of the 1960s--the moral issue was clear. The character of the country was at stake.

In this extraordinary book, Nicholas Lemann argues that a third conflict between America’s ideals and its racial realities is taking place now in the inner cities. There, a black “underclass” has become trapped in poverty while the rest of the population, white and black, averts its eyes.

How did this come to be, in a nation whose economic downturns occur from heights that few other countries ever reach? One reason is that between 1940 and 1970, more than 5 million black Americans moved from the largely rural South to the largely urban North. Poorly educated, without many useful skills, they took with them the disastrous patterns of family instability that had marked their lives as sharecroppers.

They were cast out of the South by a succession of economic and technological changes over which they had no control. Lemann begins his book with the first of these: the demonstration, on a farm near Clarksdale, Miss., in October, 1944, of a mechanical cotton-picker that quickly made armies of field hands redundant. There was still a need for people to chop weeds around the cotton plants, but in 1967, the federal minimum wage was extended to farm labor, and planters replaced choppers with chemical defoliants.

In search of jobs and decent treatment, many black families traveled to Chicago. The last stop on the Illinois Central line, it seemed a mecca of good pay, lively entertainment, friends from home, subsidized housing, and support from a political system, provided you were loyal to it.

In time, jobs for unskilled workers began to dry up in Chicago. Housing officials who had given decent shelter to reputable black families began conducting elaborate investigations of applicants, including an office interview by a social worker, employment verification, a police-record check and a home visit.

By the early 1980s, none of this was required. The city government had long since renounced its role as watchdog over the high-rises, which now housed thousands of desperately poor, mostly female-headed families. The dark buildings were controlled by gangs and infested with drugs. Children were intimidated, truant, likely to shoot and be shot. The projects had become, says Lemann, among the worst places to live on Earth.

His book is about three cities: Clarksdale, Miss., the home of paradigm families; Chicago, where those families moved over the years; and Washington, where a passionate effort was made in the 1960s to change the conditions of life for families like these. It is a measure of Lemann’s achievement that “The Promised Land” is as good in describing the sordid environment of the Chicago projects as it is in recounting the ideological wars about fighting poverty that were waged in the Kennedy and Johnson Administrations.

Poverty has been on the presidential agenda since at least the New Deal, but its racial aspect was of no special concern to Franklin Roosevelt, nor, says Lemann, to writers such as John Kenneth Galbraith, whose “The Affluent Society,” published in 1958, mentions it only once. In the early 1960s, race as a political issue meant George Wallace and Bull Connor and the system of officially sanctioned discrimination that they represented. Its identification with inner-city ghettos in the North, with poverty, crime and illegitimacy, was still to come.

When it did, and the federal government set about to reverse the steady decline in living standards among the urban poor, poverty and its racial ingredient became the province of the Office of Economic Opportunity. Lemann reports the history of the anti-poverty campaign not as a dreary bureaucratic tale but as an important clash of ideas: between those who wished to acculturate the poor into the folkways of the middle class and those who sought to arrange for the poor to seize political power through community-action programs. Ironically, the two may be said to have merged in many cities. Community-action agencies, flying the banner of “maximum feasible participation” of the poor, became large-scale employers, with tens of thousands of salaried workers. Thus a government-dependent middle class came into being, one whose goal was to fight poverty, and who accomplished it at least in their own cases.

Such agencies were envisioned as radical alternatives to the traditional bureaus that served the caring and teaching roles of government. They were meant to streamline the delivery of job training, early childhood education and health care. Their results were paradoxical at best:

“The many people who got jobs in the program,” Lemann writes, “were helped enormously, while the neighborhoods that were supposedly being healed got worse, not least because many of their residents used their new government paychecks to finance their relocation to better areas”--to “clear,” in ghetto parlance.

“Within a couple of years of its birth,” Lemann writes, “community action had the reputation of being not only a black program--the perception that Shriver had wanted so badly to avoid--but a black radical program.”

America’s rejection of the anti-poverty program’s attempts to speed the rise of the black underclass helped create modern conservatism, Lemann writes.

“The great migration delivered the coup de grace to the Democrats as a presidential party: It hastened the movement of millions of middle-class white voters to the Republican suburbs, and it caused millions more blue-collar voters who didn’t move to stop voting for the Democratic candidate for President.”

In the late 1960s, Martin Luther King searched in vain for a magic formula that would reach the nation’s conscience about the urban ghettos. Lemann’s book helps us to see why. Slaves were innocent victims. So were Rosa Parks and Emmett Till, a century after the Civil War. But what of the descendants of Ruby Haynes, and the other Mississippi migrants whose stories Lemann tells in “The Promised Land”? What of men and women who produce generation after generation of illegitimate children? Who choose crack and cocaine? Who will not seek or sustain work, but look to the government--that is, to those who will--for the means to continue such lives? Are they innocent? Should they invoke our sympathy as victims?

There are any number of explanations, and few confident solutions. Lemann reminds us that a number of Great Society programs, such as Head Start, have been quite valuable to poor families, and that other government-assisted programs are needed to train and counsel young people in the inner cities. Though there is no new Emancipation Proclamation or Civil Rights Act that will bring a sudden end to the misery, there are means of help, of which determined and disciplined people can make use.

Ultimately, the innocence of persons such as those chronicled in “The Promised Land” may not matter very much. What matters most is what kind of country the rest of us are content to live in. The conclusion of this extremely responsible, clearly written and absorbing book gets it right:

“Programs for middle-class blacks--affirmative action and minority set-asides--are never going to set the country aflame with a sense of righteous purpose. Neither will family allowances or an increase in the Earned Income Tax Credit. The conditions that now prevail in the ghettos, honestly presented and openly discussed, could.

“To be born into a ghetto is to be consigned to a fate that no American should have to suffer. The more clearly we can be made to see that and to understand the causes of the situation, the less likely it will be that we will let it stand.”