As the dispirited Army of the Potomac moved north from Washington, D.C., in early September 1862, few who marched in its ranks or served in the government on whose behalf it fought were confident it could accomplish the mission before it. After more than a year of stinging defeats, ill-conceived maneuvers, and interminable inaction, that army's harried, defensive posture contrasted sharply with the bold confidence of the Army of Northern Virginia under Robert E. Lee.

Lee surmised the Union army was "much weakened and demoralized," but he also knew that unless his own army soon won several decisive victories, the North would eventually wear down the Confederacy and win the war. So, Lee struck swiftly through Union-occupied Maryland, wagering that he might thereby tip the balance and hasten French and British recognition of the Confederacy. If his military exploits could effect that diplomatic coup, both the war and the union might quickly come to an end.

Much, then, was riding on the fortunes of the Army of the Potomac as it rolled into Frederick, Maryland, on the morning of September 10. As the 27th Indiana Regiment stopped for rest outside town, Corporal Barton Mitchell noticed a large envelope lying in the grass. Picking it up, he discovered it to be a sheet of paper wrapped around three cigars; on the wrapper was a hand-written heading, "Headquarters, Army of Northern Virginia, Special Orders, No. 191." The document was dated September 9.

Corporal Mitchell held in his hand a typically cunning and complex set of battle plans drawn up by General Lee. Of the seven copies of this secret document, six found their way to Confederate commanders scattered north and west of Washington, while the seventh inexplicably came to rest in that field outside Frederick.

"The odds against the occurrence of such a chain of events must have been a million to one," writes James McPherson in his elegant study, Crossroads of Freedom: Antietam. "Yet they happened." And because they did, the course of the war and the structure of American society were changed unalterably. As a member of Lee's staff at Antietam later remarked, "The loss of this battle order constitutes one of the pivots on which turned the event of the war."

In the short term, the fortuitous discovery proved crucial in the hasty round of Union planning that stole the element of surprise from Lee. In receiving Lee's plans, General George McClellan, the mercurial commander of the Northern Army, "was granted a windfall such as few generals in history have enjoyed." Only seven days after Corporal Mitchell s discovery, the battle was joined at Sharpsburg on the banks of Antietam Creek. Here, in a hellish day of conflict, the opposing armies struck at each other ruthlessly and relentlessly….

"Despite the ghastly events of September 11, 2001," writes McPherson in his book's opening sentence, "another day 139 years earlier remains the bloodiest single day in American history." The 6,300 to 6,500 soldiers killed or mortally wounded that day were more than twice the number killed on September 11 and more than died in all the other wars fought by America in the 19th century combined; that includes the War of 1812, the Mexican-American War, the Spanish-American War, and all the Indian Wars.

Nevertheless, in McPherson's account, the beneficial consequences of Antietam were as great as its costs were steep. The most important consequence became evident five days after the battle, when Abraham Lincoln addressed his Cabinet. He told them that shortly before the battle at Antietam he had made "a promise to myself and [hesitating a little] to my Maker" that "if God gave us the victory in the approaching battle, [I] would consider it an indication of Divine will" in favor of emancipation. Lincoln read Antietam as God's sign that "he had decided this question in favor of the slaves," and that same day he issued his proclamation declaring that unless the states in rebellion returned to the Union by January 1, 1863, their slaves "shall be then, thenceforward, and forever free."

The Emancipation Proclamation, in McPherson's words, irrevocably "changed the character of the war." Ruling out all possibilities of an armistice that would leave slavery in place, it turned a war to preserve a political union into a crusade to eliminate an abominable practice. The change galvanized public opinion.… As Horace Greeley put the matter in an editorial on September 23, the Emancipation Proclamation "is the beginning of the end of the rebellion; the beginning of the new life of the nation." Indeed, Greeley concluded, it marked "not only an era in the progress of the nation, but an epoch in the history of the world. …"

All this "came as a result of a wrapper wound around cigars in a Maryland field."