I am reading McClellan's War by Ethan Rafuse and came across something interesting and relevant to this discussion. On page 315, Rafuse says that Sumner arrived around 0600 and was told "to his intense dismay" that McClellan wanted the Second Corps to remain where it was. Hooker was having success against the enemy's left at this point, so McClellan sent an aide to Burnside to tell him to prepare to attack the enemy's right. The Second and Fifth Corps were to be used to either attack the center if the flank attacks were successful, or provide support to "counter a Confederate counterstroke."
Going through Rafuses references:
Ruggles to Sumner, September 16, 1862, OR, vol. 51. pt. 1: 839; (
link) is McClellan's orders for Sumner sent 1750 hrs on the 16th. It includes orders for Sumner to place his artillery over the creek on the 16th (which I think Marion gives to Sumner's own initiative) and for the corps to be ready to march "an hour before daylight". A second order stamps 1930 hrs tells Sumner to establish two batteries behind general HQ.
Strother, “Personal Recollections,” 281; (
link) is Strothers account of Antietam in his diary. He found McClellan at the Newcomer House, and around 0730 hrs McClellan and aides ascended the hill behind the house to watch Hooker's advance. A little later McClellan rides off to the "same commanding knoll" he'd set his CP up on the 15th; this is the knoll next to the Ecker house overlooking the middle bridge.
Samuel S. Sumner, “The Antietam Campaign,” in Civil War and Miscellaneous Papers, vol. 14 (Boston: Military Historical Society of Massachusetts, 1918), 10; discussed above.
Oliver Otis Howard, Autobiography of Oliver Otis Howard, Major General United States Army, 2 vols. (New York: Baker and Taylor, 1907), 1: 291; (
link) is Howard's account and basically says he didn't receive an order to march until 0720 (this is probably taken from Sumner's report). He does add a tale that on the night of the 16th some of the HQ wagons rocked and were driven off by artillery.
Marion V. Armstrong, “A Failure of Command? A Reassessment of the Generalship of Edwin V. Sumner and the Federal II Corps at the Battle of Antietam,” in Leadership and Command in the American Civil War, ed. Steven E. Woodworth (Campbell, Calif.: Savas Woodbury Publishers, 1995), 75– 76; is Armstrong's interpretation, but he adds two new primary sources - Page's History of the 14th CT (
link) which says they were woken up at 0200 hrs, and Sumner's testimony to the JCCW (
link), where he says the same as his report (also referenced,
link), that he didn't receive an order to advance until 0720.
Sears, George B. McClellan, 305; is Sears' take on Sumner's tale, and he goes far further than the source material would indicate - this references the same SS Sumner to GB Davis 4th April 1897 letter discussed above.
McClellan, McClellan’s Own Story, 603; (
link) is McClellan discussing Burnside's failure to move.
Burnside to Williams, September 30, 1862, OR, vol. 19, pt. 1: 419; (
link) is Burnside's report.
The one piece of cited primary evidence we don't have intimate access to is the Sumner to Davis letter, and it is the lynchpin that the Sears type argument is founded upon. All we have is fragmentary quotes, the fullest of which is in Armstrong:
"The artillery could be heard plainly on the right, and General Sumner was so impressed with the necessity of reaching the field of battle that he, personally, rode over to the army headquarters hoping to facilitate the movement of his command... I don’t know if General McClellan was asleep or engaged inside. I know General Sumner was uneasy and impatient….We remained outside Headquarters for some time, quite an hour or so as I remember it"