One hundred and fifty years after the guns began shelling Fort Sumter this April, Americans remain fascinated with the Civil War. Why do we care about a war that ended so long ago?

Part of the answer lies in the continental scope of a conflict fought not on some foreign land but on battlefields ranging from Pennsylvania to New Mexico and from Florida to Kansas, hallowed ground that Americans can visit today. The near-mythical figures who have come to represent the war intrigue us still: Abraham Lincoln and Robert E. Lee, Ulysses S. Grant and Thomas J. “Stonewall” Jackson, William T. Sherman and Nathan Bedford Forrest, Clara Barton and Belle Boyd. Most important, the sheer drama of the story, the momentous issues at stake, and the tragic, awe-inspiring human cost of the conflict still resonate. More than 620,000 Union and Confederate soldiers gave their last full measure of devotion in the war, nearly as many as the number of American soldiers killed in all the other wars this country has fought—combined. Americans in both North and South were willing to fight on, despite such horrific casualties because their respective nations and societies were at stake.

Would America move toward a free-labor capitalist economy and a democratic polity in all regions, or would a slave-labor plantation economy and a hierarchical society persist in half of the country? The war of 1861–1865 resolved two festering questions that the Revolution of 1776 and the Constitution of 1789 had left unresolved: whether this fragile republican experiment called the United States would survive as one nation, indivisible; and whether this nation born of a declaration that all men are created with an equal right to liberty would persist as the largest slave-holding country in the world. Many Americans, painfully aware of the unhappy fate of most republics through history, worried whether theirs would also be swept into the dustbin of history.

Before the Civil War, some Americans had advocated the right of secession and periodically threatened to invoke it; eleven states did invoke it in 1860–61. But, since 1865, no state or responsible political leader has seriously threatened secession, not even during the “massive resistance” to desegregation from 1954 to 1964, years of tension that overlapped and in some ways overshadowed the Civil War’s centennial. In 1854, Abraham Lincoln said that the “monstrous injustice of slavery . . . deprives our republican example of its just influence in the world—enables the enemies of free institutions, with plausibility, to taunt us as hypocrites.” Since 1865, that particular “monstrous injustice” has existed no more.