And if people took this bounty for granted, were too busy to stop and look, that's where artists came in, as they regularly did in the first half of the 19th century, to advertise the New Eden: illuminate it, enhance it, edit out flaws, say in paint, "This is ours!" In front of panoramic pictures by the likes of Alfred Bierstadt and Frederic Church even citizens who rarely ventured beyond city parks or their own backyards lingered, enthralled.

The enthrallment didn't last much into the second half of the century. And for Brown it lasted only hours. He made his comment to the jailer who was taking him, in an open cart, to the gallows to be hanged for murder, treason and inciting slave rebellion after his Harpers Ferry raid. With his death, which many consider a spark that fired the Civil War, the American landscape began to change, topographically, politically and psychologically.

That change is the subject of a perspective-altering show called "The Civil War and American Art" at the Smithsonian American Art Museum, one of many events marking the 150th anniversary of the explosive 1863 arrival, during the Civil War, of the Emancipation Proclamation. On the surface the show looks straightforward, even ordinary. We know most of the paintings and photographs; they're classics. What we know less well is their meaning within the context of the nation's single greatest internal catastrophe, and that's what we learn here.

Within little more than a year of Brown's death the entity called the United States began to lose its shape. Southern states seceded; war began. In 1862 the Shenandoah countryside that had inspired Brown's brief reverie became part of a new, breakaway state of West Virginia. Before long the farmland he saw was a battlefield, its crops trampled, its trees burned.

He wouldn't have been surprised. He well understood the path that his death would set history on. But now other eyes, some more objective than his, were surveying the fast-altering American terrain. Journalists were embedding themselves in the thick of fighting and sending back reports. After battles photographers like Mathew Brady, Alexander Gardner and Timothy O'Sullivan lugged their cameras onto blood-soaked fields to record the destruction. Sometimes, in the name of art, which is what they considered their work to be, they took liberties with reality, shifting the corpses of soldiers around to get more effective shots.

Even poets got into the act. Herman Melville stayed physically out of the fray but wrote dozens of knotty, passionate poems during, and about, the war. (Brown's execution was one of his subjects.) Walt Whitman worked as a nurse in military hospitals, keeping a diary that would eventually become "Specimen Days." And in far-off Amherst, in the spring of 1862, Emily Dickinson attended a memorial service for a friend, Frazar Stearns, killed by a Confederate bullet, then went straight home and started some of her greatest work.

But what about our painters, those who had turned the American landscape into a national symbol? From accounts in art history books past, you would think they were missing in action throughout the war, mute, disengaged, in retreat. The Smithsonian show, organized by Eleanor Jones Harvey, senior curator at the American Art Museum, argues otherwise. Artists were on the job, but working at a disadvantage.

Landscape painting was the American version of European history painting, and history painting dealt in heroics — the heroically good, the heroically bad. Neither description applied to the Civil War, a grim, down-and-dirty fratricidal face-off. Overnight, it seemed, the patented uplift of Hudson River School paintings — pictures empty of people and glowing with grace — was out of step with history.

Some painters tried to adjust by incorporating the subject of war obliquely and strategically, as Church did. The 1861 public debut of one of his big showpieces, "The Icebergs," almost exactly coincided with the declaration of war. Bad luck. He had to act fast if he wanted to generate market spin, so he gave the painting a new title, "The North," a suddenly newsy Union label. Later, when he took the picture to England, where there was sympathy for the Confederate cause, he changed the name back.

Most of Church's Civil War-period paintings seem to work that way, interpretively, through context. The 1862 "Cotopaxi," which depicts a South American volcano in blood-red eruption, was painted the year after Frederick Douglass, in a powerful speech, called American slavery "a moral volcano." Was "Cotopaxi" a response to Douglass? All we know for sure is that eruption was very much in the air.

At least one Church picture was an immediate reaction to war news. After Confederate forces overcame a Union army detail at Fort Sumter in South Carolina, Church turned out a little rush-job called "Our Banner in the Sky," showing clouds and stars in one of his signature twilight skies taking the form of Sumter's tattered Union flag.

The picture, coming at the start of the war, was a hit and gave Church's career a nice bounce. But the kind of painting he specialized in was basically on its way out. War and photography killed it. The fashion for big, glowy, escapist Edens evaporated behind a growing onslaught of photographic images of appalling human slaughters at places like Shilo, Antietam and Gettysburg.

The Smithsonian show has a selection of such photographs, as does a concurrent exhibition, "The Civil War in America." They are magnetic; you can't avert your eyes any more than 19th-century Americans could. This was the new unromanticizable reality. If painting was going to compete, it too had to get real, and it did with younger artists like Winslow Homer and Eastman Johnson.

Homer was a commercial illustrator sent to the front by magazines and newspapers to make on-the-spot drawings of combat and camp life. Still in his 20s, he was also a brilliant, largely self-taught painter who reworked some of the subjects as oil sketches, going for psychological subtlety.

These taut little pictures are astonishing, acutely observed wartime vignettes: a recruit listening, eyes down, to distant music; a stiff young officer confronting hostile prisoners of war. Utterly modern, they have the gravity of classical drama. Landscape is still there, but it's the terrain of a changed America: ex-Eden, flat, scarred, unfruitful.

During the same years that Homer was focused on front-line combat, Johnson, older and with a well-established art career, was turning his attention to another aspect of the national conflict: African-American life, which receives engaged, regardful treatment in his hands. A painting called "A Ride for Liberty — The Fugitive Slaves, March 2, 1862," with its near-silhouette image of a family fleeing on horseback, stands as a bold graphic emblem of the era. Another, the 1859 "Negro Life at the South," is a visual dissertation on the complexities of class and skin color.

This earlier picture in particular, with its multiple figures and pairings in a domestic setting, bursts with information, sorted out at length in Ms. Harvey's fine catalog. Although a couple of readings in her book feel a bit overstretched, that's not true in this case. Ms. Harvey finds Johnson telling an extremely complicated sociopolitical story and the evidence, all of it, is there.

Homer painted black people too, but mainly after the war, during the bitterness of Reconstruction. He is as empathetic a commentator as Johnson, but a better, more imaginative artist. His 1877 "Dressing for the Carnival," with its image of a young black man being sewn into a bright-colored, shimmering costume to participate in a street festival, is one of the great American paintings. And the outstretched arm of the figure standing behind the reveler is one of the great American arms: strong, long, directive, poised to embrace.

As self-assured as Homer's art feels, he was badly unnerved by the war. "He came home so changed that his best friends did not know him," his mother wrote. I wonder if the lone figure in the painting called "The Veteran in a New Field," done just after the war, is in some sense a self-portrait.

The man stands with his back to us in his shirt sleeves, Army coat tossed aside. He bends over an old-fashioned scythe, facing a wall of ripe summer wheat almost as tall as he is. Some historians take him as Death, cutting down grain as the war did lives. Others see a symbol of restoration, of getting back to life, however daunting the prospect. But it's also possible, if you can put aside thoughts of war, death and striving, to see the painting, as, above all, a landscape, and a beautiful one when you stop to look, done in blue and gold, all but abstract, with areas of gold crackling with light as if energy charged.

You also realize that if the man put down his scythe and stood up straight, he could probably look over the wheat, and beyond it. What America would he see? One brighter, wiser, freer? One that justified the trials he had endured, or that asked for more? Will what's in front of him and behind him be the future story: wheat fields alternating with battlefields to the horizon?

Impossible to say, of course. But just thinking such thoughts, asking and wondering, and looking afresh, is what this show is about.