September 17, 1862, the day of the Battle of Antietam, was one of the most consequential days in the history of the United States and a moment in time when the future of the country could have veered in two starkly different directions. The fighting near Sharpsburg, Maryland would change the course of American history, but in the process, it became the costliest day this nation has ever known, with more than 23,000 men falling as casualties. AUTHOR: Daniel J. Vermilya is a Civil War historian who works as a park ranger at Gettysburg National Military Park. He has previously worked as a park ranger at Antietam National Battlefield and Monocacy National Battlefield and is also a licensed battlefield guide at Antietam. He is the author of The Battle of Kennesaw Mountain and James Garfield and the Civil War. 150 images and maps