Dennis Frye, a former chief historian at the Harpers Ferry National Historical Park, has posted an informative, insightful article on how George McClellan outwitted Robert E. Lee at Antietam--actually, not just at Antietam but in the Maryland Campaign as a whole. Frye discusses some interesting little-known facts and addresses a few of the common myths about McClellan's performance at Antietam. Face Facts: 'Little Mac' Outwitted Lee at Antietam http://www.historynet.com/no-small-deed-face-facts-little-mac-outwitted-lee-antietam.htm Here's an excerpt: McClellan didn’t know Lee’s thinking, but as a strategist he certainly could surmise Lee’s intentions. McClellan had divined Pennsylvania to be a primary Confederate target since the invasion’s outset. He knew Lee was concentrating at Sharpsburg, and he could interpret this two ways: Either Lee expected to fight there, or the Confederate commander intended to move north from there. If McClellan were to capture the Sharpsburg–Hagerstown Turnpike and block Lee’s avenue northward, he could eliminate the “move” option, and produce an outstanding outcome—defeating the Rebel army’s invasion. The Union commander seized the occasion. By afternoon and evening of September 16, McClellan had moved nearly 23,000 soldiers from his 1st and 12th Corps—about one-third of his total force—around the Confederate left flank. And they took the road! Consider McClellan’s wise preemptive action. About two miles north of Sharpsburg, he had blocked Lee’s line of advance into Pennsylvania, and had done so without a battle, indeed barely firing a shot! McClellan’s prescient move on September 16 constituted the demise of Lee’s invasion strategy. The most famous Southern general of the war had been outsmarted, outflanked, and outmaneuvered by the most harshly criticized Union general of the war.