Library of Congress via AP This 1862 photo made available by the Library of Congress shows soldiers next to a lone grave after the Battle of Antietam near Sharpsburg, Md.

When dawn broke along Antietam Creek on Sept. 17, 1862, cannon volleys launched a Civil War battle that would leave 23,000 casualties on the single bloodiest day in U.S. history and mark a crucial pivot point in the war. And yet it might never have occurred - if not for what a historian calls a "freakish" twist of fate. Days earlier, a copy of Gen. Robert E. Lee's detailed invasion orders, wrapped around a few cigars, accidentally fell in a farm field and were discovered by Union infantrymen who passed their stunning find up the chain of command, spurring action.

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Library of Congress via AP Dead Confederate soldiers in a ditch after the Battle of Antietam near Sharpsburg, Md.

Library of Congress via AP The front side of Confederate Army Gen. Robert E. Lee's Special Order No. 191 dated Sept. 9, 1862. The handwritten document detailed the Southern commander's audacious plans for an invasion of enemy territory that would propel the Confederates to victory. Carelessly left behind as Lee's army marched north, the copy was spotted in a field by Union infantrymen and relayed up the North's chain of command.

Library of Congress via AP The back side of Confederate Army Gen. Robert E. Lee's Special Order No. 191 dated Sept. 9, 1862.

Library of Congress via AP President Abraham Lincoln and Gen. George B. McClellan sit in the general's tent after the Battle of Antietam near Sharpsburg, Md, in 1862. McClellan's skill in organizing and preparing troops was what made Lincoln elevate him to command, even though the president had long been frustrated by another defining trait of "Little Mac" - his paralyzing deliberateness and tendency to grossly exaggerate the forces he faced. As a general, he was the temperamental opposite of Gen. Robert E. Lee.