The Federal Army of the Potomac

George McClellan’s Army of the Potomac consisted of 87,000 men organized into six army corps with a total of eighteen infantry divisions, along with a cavalry division.

The Confederate Army of Northern Virginia

Robert E. Lee’s Army of Northern Virginia consisted of 45,000 men organized into two commands with a total of nine infantry divisions, along with an artillery reserve and a cavalry division.

Why did Lee have Commands instead of Corps?

Lee’s organization of his army into commands – sometimes referred to a “wings” – was because the Confederate government had not yet authorized the use of army corps. Lee recognized that nine divisions were too many subordinate organizations for anyone to effectively command, and gave Longstreet and Jackson each command of a group of divisions as the army’s two senior division commanders.

The solution was not perfect. Both generals were still technically division commanders, so to free them up their senior brigadier generals had to take over their divisions. Which meant their senior colonels had to take over their brigades. In an army already short of senior officers and soon to lose even more, this command structure was dangerously thin. The Confederate Congress authorized the creation of army corps during the Maryland campaign, and by November Longstreet’s and Jackson’s Commands had become the First and Second Corps.