September 17, 1862, was the bloodiest single day in U.S. history. An estimated 6,300 Union and Confederate soldiers died at Antietam, Maryland, in a savage battle that took place nearly a year and a half into the Civil War. It was one day in a war that raged from 1861–65 and cost some 623,000 lives. In a total national population of twenty-seven million in 1860, that number would be proportionately equivalent to losing more than five million today. That bloody day marked the seventy-fifth anniversary of the signing of the Constitution.

At stake in the Civil War was the survival of the United States of America as a single nation. Eleven Southern states, invoking the spirit of 1776, seceded from the Union in 1861 to form a nation they named the Confederate States of America. The Federal Government refused to allow it. Massive armies representing the Union and the Confederacy squared off in a conflict that tested the experiment in self-government as never before. At the end of the Civil War's carnage, the primacy of the Federal Government over the states was indisputably upheld.

Americans had been wrestling with the fundamental question of nationhood since the earliest days of the Revolution. In 1774, as the British colonists struggled to unite in the cause of American liberty, Patrick Henry rose to address the Continental Congress in one of its earliest sessions: "The distinctions between Virginians, Pennsylvanians and New Englanders are no more. I am not a Virginian, but an American." It took the Civil War to make it so.