At the outbreak of the Civil War Alexander Gardner was appointed to General George McClellan's staff with the honorary rank of captain. Initially he and a small corps of photographers copied maps and charts for the Secret Service, which were distributed as photographic prints to both field and division commanders. For two years, while he retained his position as manager of Mathew Brady's Washington, D.C., studio, Gardner worked as a field photographer. He left Brady in November 1862 and established his own business, taking with him many of Brady's most experienced staff. The Battle of Antietam took place on September 17, 1862. On a small creek near Sharpsburg, Maryland, the Army of the Potomac met the advancing Army of the Confederacy. Although more than twenty-six thousand soldiers were killed or wounded in fierce fighting, Robert E. Lee's first invasion of the North was soundly repelled by McClellan and the Confederate general was forced back into Virginia. Long considered the only known Civil War view of a battle in action, Gardner's photograph actually shows reserve artillery east of Antietam the day after the battle. What was thought to be the smoke of guns covering the fields in the center and right distance is fog or early morning mist. During the Civil War most photographers worked with the collodion-on-glass negatives, which required delicate and laborious procedures even in the studio. When the photographer was ready for action, a sheet of glass was cleaned, coated with collodion, partially dried, dipped carefully into a bath containing nitrate of silver, then exposed in the camera for several seconds and processed in the field darkroom tent--all before the silver collodion mixture had dried. Given the danger of their situation and the technical difficulty of their task, front-line photographers rarely if ever attempted action scenes. Printed from one half of a stereo negative, this small view served as a memorial to the single bloodiest day of the war.