The common assessment of George Brinton McClellan on the part of Civil War historians is that he had the slows. Great organizer; impressively looking commander; endeared himself to his men by his care for them during difficult times. But, when it came time to put his troops into battle, he lacked the moral courage to do so with speed and force. The Peninsula Campaign is a classic example, where his creeping advance on Richmond ultimately failed when the opposing general, R.E. Lee, took command and took the offensive.
So it is a real discovery in Michigan at Antietam: The Wolverine State's Sacrifice on America's Bloodiest Day to see how McClellan moved with expedition once he had the Lost Order in hand, verified, proven, incontrovertible. This is not the classic McClellan of most Civil War histories but the doppelganger who, within several hours, figures out a plan and puts his army on the move to execute it.
A key part of this discovery is the timeline. All of the accounts of the Lost Order that form the basis for the account in Michigan at Antietam demonstrate that the document made its way to McClellan by noon. To ensure the enemy movements contained in the Order were still operative, McClellan dispatched his cavalry commander on a reconnaissance: “The following order of march of the enemy is dated September 9. General McClellan desires you to ascertain whether this order of march has thus far been followed by the enemy.” Pleasanton did, in fact, ascertain it had been followed. Receiving Pleasonton’s confirmation that Confederate dispositions corresponded to the document that afternoon, the Union general made his plans. He sent an order at 6:20 p.m. (around sunset) for an advance the next morning at daybreak. (And then telegraphed Lincoln at midnight about what had happened, not at noon as some histories have mistakenly reported. The timeline makes sense as it is laid out in Michigan at Antietam.)
The result was the Battle of South Mountain on September 14, which would never had happened if the Lost Order had not been found, verified, and forwarded to McClellan, who then took uncharacteristic boldness in moving out after his nemesis Lee.
And without South Mountain there would have been no Battle of Antietam.
And without Antietam … well, you already know the rest of the story as it developed. Know, that is, if you get a copy of Michigan at Antietam and contribute thereby to erecting a monument to the brave Michiganders who were there.