Editing George B. McClellan (section)

====Battle of South Mountain====
At the discovery of the Lost Order, McClellan's Assistant Adjutant General verified the signature and handwriting of the officer who wrote out the order, as he knew him well, so there was no doubt as to its authenticity.<ref>[[John Keegan|Keegan, John]] (2019) ''The American Civil War'' New York: Knopf. p. 167. {{isbn|978-0-307-26343-8}}</ref> Within hours of receiving the order, McClellan dispatched some of his cavalry to assess whether the Confederates had moved in accordance with the order.<ref name=murfin162 />

Still, historians—including [[James M. McPherson]] in ''Crossroads of Freedom: Antietam'' and ''[[The Battle Cry of Freedom]]'', [[Stephen Sears]] in ''Landscape Turned Red'',  [[John Keegan]] in ''The American Civil War'', and James V. Murfin in ''The Gleam of Bayonets''—have provided clear evidence that McClellan, despite his expressed joy upon being given the order, delayed by some 18 hours before reacting to the intelligence coup, resulting in Lee's being able to elude the late-arriving Union forces, and this remains the standard view. However, Gene Thorp in a 2012 article in ''[[The Washington Post]]'' cited evidence that the vanguard of Army of the Potomac was in motion all day on the 13th due to orders McClellan had issued the previous day.<ref>Thorp, Gene (September 7. 2012) "In defense of McClellan at Antietam: A contrarian view" ''The Washington Post''</ref>  After the war, McClellan held to the claim that he acted immediately to put his armies on the move.<ref name=murfin162>Murfin, James V. (2004) [1965] ''The Gleam of Bayonets'' Baton Rouge: Louisiana State University Press. pp. 162–63. {{isbn|978-0-8071-3020-9}}</ref>
[[File:Battle of South Mountain.jpg|thumb|Battle of South Mountain by [[William Waud]]]]
McClellan ordered his units to set out for the South Mountain passes and was able to punch through the defended passes that separated them from Lee. The stubborn Confederate defenses gave Lee enough time to concentrate many of his men at [[Sharpsburg, Maryland]]. As noted by historians such as Stotelmyer, the significance of the Union victory at South Mountain should not be underestimated. It ruined Lee's plans to invade Pennsylvania and took the initiative away from the Confederate commander. The Battle of South Mountain also presented McClellan with an opportunity for one of the great theatrical moments of his career, as historian Sears describes:{{blockquote|The mountain ahead was wreathed in smoke eddies of battle smoke in which the gun flashes shone like brief hot sparks. The opposing battle lines on the heights were marked by heavier layers of smoke, and columns of Federal troops were visible winding their way up the mountainside, each column ... looking like a 'monstrous, crawling, blue-black snake' ... McClellan posed against this spectacular backdrop, sitting motionless astride his warhorse Dan Webster with his arm extended, pointing Hooker's passing troops toward the battle. The men cheered him until they were hoarse ... and some broke ranks to swarm around the martial figure and indulge in the 'most extravagant demonstrations'.<ref name="qaFQ3" />}}

The Union army reached Antietam Creek, to the east of Sharpsburg, on the evening of September 15. A planned attack on September 16 was put off because of early morning fog, allowing Lee to prepare his defenses with an army less than half the size of McClellan's.<ref name="dXxot" />