The greatest poem about the Civil War doesn’t mention North or South, slavery or secession. Walt Whitman knew about all these things, of course; many of the poems in “Drum-Taps,” the collection he published a few months after Appomattox, address the political and military realities of the war head-on. But “When Lilacs Last in the Dooryard Bloom’d” leaves them out of sight. Even when Whitman describes the funeral procession of Abraham Lincoln, which wound through the country in the spring of 1865, he takes care not to mention the president’s name: Lincoln is referred to only as “him I love,” or else reduced to “a corpse” or “a coffin.” In writing about the Civil War, Whitman seems to say, his true subject, his only possible muse, is death itself.

Whitman, who spent the war years as a volunteer nurse in Washington, D.C., had seen as much death as almost anyone. In his poem “The Wound-Dresser,” he writes with brutal directness about “the crush’d head,” “the neck of the cavalry-man with the bullet through and through,” “the stump of the arm, the amputated hand.” In “Lilacs,” however, Whitman devotes all the power of his liquid, rending lines to convincing the reader that death is more than just the ruin of the body. Death, we are willing to believe while the poem lasts, is “lovely and soothing,” “delicate,” “cool-enfolding,” a “dark mother,” “a deliveress.” The poet imagines himself “with the knowledge of death as walking one side of me, / And the thought of death close-walking the other side of me, / And I in the middle as with companions, and as holding the hands of companions.”

Not until I read “This Republic of Suffering” (Knopf, 368 pp., $27.95), the astonishing new book by Drew Gilpin Faust, did the full power of Whitman’s poem become clear to me. The work of “Lilacs” was to reconcile a nation to death, and if the subject inspired some of Whitman’s most beautiful writing, it was only because unprecedented beauty was needed to overcome the unprecedented horror of the Civil War. “No one expected what the Civil War was to become,” Ms. Faust writes at the beginning of her book, and it is the terrible surprise of the war, the inability of Americans to predict or prepare for its cost, that she so powerfully communicates. Like the Union doctor at Gettysburg who found himself charged with burying thousands of corpses even though he “had not a shovel or a pick,” Civil War-era Americans were forced to improvise ways of coping with a slaughter they never expected.

When the war began with the shelling of Fort Sumter, in April 1861, both the Union and the Confederacy expected that it would be over quickly. Ms. Faust quotes a South Carolina senator who promised “to drink all the blood that might be shed as a result of the Confederate declaration of independence.” By the time Lee surrendered four years later, 3 million Americans had served in the two armies, and 600,000 had perished — more than in all other wars in American history put together. If the same percentage of Americans were to die in a war today, Ms. Faust points out, it “would mean six million fatalities.”

The unexpected scale of the war meant that its every aspect, from battlefield tactics to grand strategy, had to be improvised on the spot. As Ms. Faust shows, the same was true of the way Americans dealt with death. The challenges were both material and psychological. As armies and governments tried to figure out how to bury so many corpses and assign the correct name to each grave, civilians back home evolved their own rituals and fictions to try to make sense of their loved ones’ deaths in battle. Ms. Faust sheds light on both of these processes, thanks to her extensive research in official records and private correspondence. In general, she keeps her own voice muted, seldom imposing an interpretation, but allowing the dead to speak for themselves.

And speak they did. Americans of the Civil War era were generally less educated than they are today, and Ms. Faust quotes many letters whose spelling and grammar are imperfect. But they talked about the war and its costs with a seriousness and directness that, one fears, our own period could never muster. “I am aposed to one man killing another,” wrote one Union soldier quoted by Ms. Faust, but “when we are atacked and our lives are in danger by a gang of men aposed to the best government on earth I shall fight.” This ordinary citizen took it for granted that he must judge the war by his own moral code, and that the survival of his government depended on his own commitment. Such were the disciplines of democracy in the 19th century.

They were reinforced, Ms. Faust shows, by the consolations of religion. Just about every soldier she quotes took the afterlife entirely literally. It was especially important for parents and spouses, far from their loved one’s deathbed, to hear that he had died as a good Christian. Letters home from a dead soldier’s friends always dwelled on the dying man’s composure and resignation, signs that he was ready to enter the world to come. “Oh how could I of Stud it,” wrote T.J. Hodnett after his brother’s death, “if it had not bin for the bright evidence that he left that he was going to a better world.”

This did not mean, however, that patriotism and religion blinded soldiers to the human cost of the war. On the contrary, the kind of killing that took place on Civil War battlefields was shattering to Americans who had never seen war close up. As Ms. Faust writes, “infantry engagements, even as they grew to involve tens of thousands of men, remained essentially intimate: soldiers were often able to see each other’s faces and to know whom they had killed.”

Even worse than combat, perhaps, was the sight of the battlefield after the fighting ended. At the Battle of Antietam, a Union surgeon described “stretched along, in one straight line, ready for interment, at least a thousand blackened corpses with blood and gas protruding from every orifice, and maggots holding high carnival over their heads.” No wonder one Union officer wrote, “Of all the horrors the horrors of the battlefield are the worst,” or a Confederate soldier feared that what he witnessed at the Battle of Shiloh might “dethrone reason or pervert the judgment.”

Civilians back home might not have been able to imagine such sights, but they too struggled with the randomness and uncertainty of death in battle. Today, when the name of a soldier killed in Iraq is broadcast on CNN within hours, it is hard to imagine the sheer indifference of Civil War armies to the tasks of identifying the slain and notifying their kin. There were no grave registration units, no official cemeteries, and no reliable lists of casualties. Voluntary agencies like the Sanitary Commission and the Christian Commission did their best to fill the gap, but they were hopelessly overmatched.

As a result, as if in a parody of Gilded Age capitalism run amok, the burial of the dead became a thriving business. Embalmers and railroad express companies offered their services to grieving families, promising to locate their sons or brothers on the battlefield and send them home in airtight coffins. “This Republic of Suffering” reproduces the business card of one Lewis Ernde, “Furnishing Undertaker,” which promised, “Bodies taken from Antietam Battle Field and delivered to Cars or Express Office at short notice and low rates.” Thomas Holmes, an embalmer in Washington, processed 4,000 corpses during the war, at a price of $100 apiece. The poor, of course, could not afford such luxuries, and their bodies were left to decay, to be eaten by hogs, or to be desecrated by enemy soldiers.

Not until after the war was over did the government awaken to the plea of Henry Bowditch, whose son Nathaniel was killed in 1863: “If any government under Heaven ought to be paternal, the United States authority, deriving, as it does, all its powers from the people should surely be such, and should dispense that power, in full streams of benignant mercy, upon its soldiers.” Ms. Faust devotes a chapter to the story of how the postwar government, spurred on by activists like Clara Barton, undertook to locate, register, and if necessary rebury every one of the 300,000 Union soldiers killed in the war. “I hold these men,” Barton wrote, “in the light of Government property unaccounted for,” and accounting for them became a massive project, which greatly expanded the powers of the national government.

Even so, the Confederate dead were excluded, left to the voluntary efforts of local women’s associations. Appropriately, Ms. Faust — who was dean of Radcliffe until she became the president of Harvard last fall, and whose best known book, “Mothers of Invention,” deals with Southern women during the Civil War — shows how integral women were to the creation of the war’s meaning. “Let us remember,” wrote Fanny Downing of the Ladies’ Association for the Fitting Up for Stonewall Jackson Cemetery, “that we belong to that sex which was last at the cross, first at the grave.” Once again, the best gloss on “This Republic of Suffering” comes from Whitman:

I saw battle-corpses, myriads of them,
And the white skeletons of young men, I saw them,
I saw the debris and debris of all the slain soldiers of the war,
But I saw they were not as was thought,
They themselves were fully at rest, they suffer’d not,
The living remain’d and suffer’d, the mother suffer’d,
And the wife and the child and the nursing comrade suffer’d,
And the armies that remain’d suffer’d.

Mr. Kirsch’s new book, “The Modern Element: Essays on Contemporary Poetry,” has just been published by W.W. Norton.