This study investigates the development of large unit command and control in the United States Army prior to the American Civil War. It examines the results of this development in one early campaign of the war. The paper's theme is that the excessive casualties suffered during the early stages of the war were not only a result of the improvements in weapons technology, the size of the armies or the personalities of the individual commanders. Another, and potentially more serious cause was the inability of the Union commanders to command, control, and maneuver these units to achieve campaign objectives.The paper begins by describing how war is organized into three levels: strategy, operations, and tactics; and defining the concepts related to command and control. The influences on the development of the Civil War leadership are next examined. These include: Napoleonic Warfare, the teachings of Jomini, Mahan, and Halleck; the the formal and informal educational experiences of the officers. Next command and control doctrine within the Union Army is examined.The case study used for examining operational command and control during the early period of the Civil War is the Maryland Campaign of 1862 which culminated at the Battle of Antietam in September of that year. Throughout the thesis, the education and performance of the Army of the Potomac's commander, George B. McClellan is examined.The conclusion of the paper is that the United States Army was poorly prepared for the conduct of large unit operations. This poor preparation, and performance, could not be blamed on any single individual, including McClellan. It was the result of complex educational, experiential, and organizational factors which shaped the pre-war Army.Finally, this paper concludes that General McClellan's inability to decisively maneuver the forces at his disposal was a significant factor in the outcome of the engagement at Sharpsburg, Maryland on 17 September, 1862.