In the 19th century, American art was big in every sense of the word. When Hudson River School painter Frederic Edwin Church first exhibited a five-foot-by-10-foot landscape, “The Heart of the Andes,” more than 12,000 people paid admission to see it over the course of three weeks in 1859. Paintings like Church’s were typically allegories of manifest destiny, reminding American viewers of the vast, unspoiled continent that God presumably wanted them to seize. Imagine, then, how that triumphal artistic vision would accommodate the horror of the Civil War. The Smithsonian American Art Museum’s “The Civil War and American Art” show will feature war-era works by Frederic Church, Sanford Gifford, Winslow Homer, and Eastman Johnson—alongside battlefield photos by Alexander Gardner, Timothy H. O’Sullivan, and George Barnard. After the firing on Fort Sumter, Church created “Our Banner in the Sky,” an outlandish burst of pictorial nationalism in which the clouds, stars, and setting sun conspire to form a floating American (i.e., Union) flag. Other artists got a less abstract, more journalistic view of the conflict: Winslow Homer was basically embedded with the Union army, sailing down the Potomac and drawing sketches that became the basis for dozens of wood engravings for Harpers Weekly. Ultimately, in the face of the struggle for emancipation and mounting grimness on the battlefield, the landscape painter’s preferred allegorical image of America—as a divinely favored new Eden—became untenable. The exhibition promises to show how America and its art both lost their innocence.