Antietam, the bloodiest day in American history. On that single day more American casualties were sustained than in all of America’s prior wars, except for the American Revolution, combined. As for the American Revolution, the 23,000 killed and wounded at Antietam on a single day were more than one-third of the total of 58,000 Americans killed and wounded in the eight years of the Revolution.
Antietam was the culmination of Lee’s Maryland campaign. Lee had decided to enter Maryland in early September 1862 to take the pressure off war-torn Virginia, to gain supplies in Maryland and possibly recruits from sympathetic Marylanders and to inflict, if he could, punishing defeats on Union forces and, with luck, help opponents of the Lincoln administration do well in the fall elections as a result of those defeats. Go here to read a post detailing Lee’s motivation for the Maryland Campaign.
All went superbly for Lee initially in the Maryland Campaign. Supplies were abundant in Maryland. Recruits from Marylanders, while not as abundant as the Confederates would have wished, were first-rate as to quality. The Northern papers, and General Lee gained much valuable intelligence throughout the War by reading carefully every Northern newspaper he could obtain, were largely hysterical about the Confederate offensive, more than a few predicting that the War was lost. General Stonewall Jackson’s II corps was detailed by Lee to capture Harper’s Ferry, which he did on September 15, 1862 against pathetically weak Union opposition, and inflicting one of the worst defeats on the United States Army in its history, the 12,000 Union troops being the largest mass surrender of United States military personnel until the surrender on Bataan in 1942. Go here to read a post on the sorry tale.
Lincoln, desperate to stop Lee, placed Major General George B. McClellan, in disgrace after his humiliating defeat in the Peninsula Campaign, back in command of the Army of the Potomac. McClellan followed Lee in a lethargic pursuit, obviously fearful of being defeated by Lee again. The situation altered dramatically when McClellan was the beneficiary of the biggest intelligence coup of the Civil War, obtaining a copy of Lee’s Special Order No, 191 on September 13, 1862, which revealed to McClellan that Lee had divided his force and the routes that the portions of Lee’s army were to follow. Go here to read a post on the finding of the famous Lost Order. With this order in hand McClellan boasted that he would whip Bobby Lee or go home.
On September 14, 1862 McClellan attacked three gaps at South Mountain to seize them, to allow him to march over the mountain and fall on Lee’s separated units. Lee held two of the gaps after a hard day’s battle. Go here to read a post on the battle of South Mountain. With one of the gaps lost, Lee retreated and began to swiftly reassemble his Army of Northern Virginia to confront the Army of the Potomac. McClellan, inexplicably, threw away his advantage by doing almost nothing on September 15, instead of immediately following Lee in hot pursuit.
At dawn on September 17, 1862, the Army of the Potomac confronted part of the Army of Northern Virginia along Antietam Creek near Sharpsburg, Maryland. Three of the divisions of the Army of Northern Virginia were still on the road from Harper’s Ferry, marching all night to reach Lee. McClellan enjoyed more than a two to one advantage at the beginning of the battle, his 75,000 force confronting less than 30,000 Confederates. McClellan, as he did throughout the War, assumed, against all evidence, that the Confederates outnumbered him.
MClellan issued attack orders for each corps. He made no effort to coordinate attacks between the corps. With the Union advantage in numbers McClellan could have annihilated Lee’s army if he had simply had each corps get into assault position and then attack simultaneously. Instead, this very long day consisted of piecemeal attacks by individual Union corps which gave Lee the opportunity to shift his heavily outnumbered units to meet each threat in turn.