Fifty years ago, National Park Service historian Dwight E. Stinson, Jr. set out to “present a definitive study of the operations of Sedgwick’s Division at the Battle of Antietam.” The study that emerged was an internal report titled “Operations of Sedgwick’s Division.” In it, Stinson examined “certain aspects of the operations in the hope that their clarification will result in a more correct interpretation of the battle story.” These “aspects” are casualties, density of formation, accusation of rashness, the “ambush” question, and the Confederate order of battle. His sixty-page typescript report challenged the standing narrative of the fight for the West Woods established by Winthrop Palfrey’s The Antietam and Fredericksburg (New York: Charles Scribner’s Sons, 1881) and Francis A. Walker’s History of the Second Army Corps in the Army of the Potomac (New York: Charles Scribner’s Sons, 1886). While cited in some secondary sources, Stinson’s report has never been published and is located in the Antietam National Battlefield Archives. Over the next few months, selected sections of his report will be posted here. I have chosen to begin with Part III—Analysis and not post Parts I and II which Stinson calls Preliminary Data and Operations—both of which are treated in greater detail in the report's subsequent sections.
Although this study deals exclusively with the operations of Sedgwick’s Division, any analysis must consider these operations in relation to the commitment of the entire II Corps. This is necessary for two reasons: the II Corps was committed as a unit by the commanding general and the corps commander, General Sumner, was physically present at the head of Sedgwick’s Division and therefore responsible for all tactical decisions concerning that unit.
A. The Battle Plan
In McClellan’s own words, his plan for the Battle of Antietam was to attack the enemy’s left with the corps of Hooker and Mansfield, supported by Sumner’s and, if necessary by Franklin’s, and, as soon as matters looked favorably there, to move the corps of Burnside against the enemy’s extreme right, upon the ridge running to the south and rear of Sharpsburg, and, having carried their position, to press along the crest toward our right, and, whenever either of these flank movements should be successful, to advance our center with all the forces then disposable. 
It must be assumed that when Sumner received the order to advance at 7:20 both he and McClellan envisioned that the II Corps would be operating within the framework of the above stated plan. As far as General Headquarters knew, at the time Sumner was dispatched, the attack of the Federal right wing was progressing favorably.
During the hour and fifteen minutes it took the advance of the II Corps to reach the front, significant changes in the situation were taking place. Perhaps most important of all was the fact, which Sumner had no way of knowing, that Burnside’s attack was not developing as per plan, in fact would be so dilatory that the enemy would be able to shift a whole division from the IX Corps front to the northern sector and send the two reserve divisions as well. This point is usually overlooked by Sumner’s critics but the objective historian should bear in mind that Sumner acted within the guidelines set forth in McClellan’s plan and had every reason to believe that his fellow corps commanders would do so also.
To accomplish this mission, Sumner’s column contained two infantry divisions (Sedgwick and French) comprising some 12,000 men. A third division (Richardson) was delayed for a time by General Headquarters but should be considered as a part of the available force. In addition, Sumner knew that the VI Corps was en route to the battlefield and would be available for further support if needed. It was certainly not improvident to believe that an attack force of 12,000 men supported by 5,000 more had a better than average chance of breaking through the already battered Confederate left and driving it toward Sharpsburg. With this in mind, as he approached the zone of combat Sumner ordered French to follow on Sedgwick’s left.
As has been shown, this promising plan was not followed and Sumner attacked with only a third of his force. The reasons for the deviation are discussed in Section B."
 Stinson cites “McClellan, O[fficial] R[ecord], 55.” McClellan in his “Second Preliminary Report” dated October 15, 1862 recounted that “The design was to make the main attack upon the enemy's left--at least to create a diversion in favor of the main attack, with the hope of something more by assailing the enemy's right--and, as soon as one or both of the flank movements were fully successful, to attack their center with any reserve I might then have on hand.” OR—Series I—Vol. XIX/1.