Photography was still in its early days in the 1860s, and events captured during the Civil War offer a priceless window to history. When this photo was taken on Oct. 4, 1862, President Abraham Lincoln was visiting with Union Gen. George McClellan at his headquarters near Sharpsburg, Maryland. Lincoln would later remove McClellan from command after the Battle of Antietam for failing to pursue and destroy the Confederate Army under the command of Gen. Robert E. Lee. (AP Photo)