The guns have long gone silent. But the smoke lingers.

It was 150 years ago on this date—April 9—that General Robert E. Lee surrendered the Army of Northern Virginia to Union Army commander General Ulysses S. Grant at Appomattox Court House.

Its immediate significance was to end hostilities between the North and the South; and to extinguish the War for Southern Independence, or the War of the Rebellion, depending on which side you took.

The lasting significance was just as important. It put an end to talk of secession and cemented the concept of the “United States” more firmly than it had ever taken hold previously in the national mind. The nation finally fulfilled the hope of its founding fathers … but at a terrible price.

It also put an end to legalized human bondage. Slavery was America’s great original sin, a divider of people by color, by belief, and by state of residence. It took the nation’s bloodiest, most violent conflict to bring...