Wednesday, September 17, 1862
At 7:10 A.M. General Bull Sumner received orders from Union General George McClellan to cross the Antietam Creek and advance westerly across the Cornfield to the West Woods. For the last hour Sumner had watched the advance of Joseph Hooker's First Corps and Joseph Mansfield's Twelfth Corps across the Cornfield and witnessed heavy, brutal fighting which resulted in the death of Twelfth Corps General Joseph Mansfield, who was killed as he prepared his troops for advance. The Second Corps was next in line to cross the Cornfield and drive the Confederates from the West Woods.
General Sumner began the advance with two of his three divisions, leaving General Israel Richardson's division in reserve. As Sumner's troops advanced, they witnessed the carnage from the early morning fight in the Cornfield. Sumner ordered his lines to close and advance quickly across the Cornfield into the West Woods. The close lines would prove to be problematic and would have devastating consequences.
The Twentieth Massachusetts was on the left flank of the second line of battle. As they approached the West Woods, the Thirty-Fourth New York Regiment, immediately in front of the Twentieth, was dispatched south to the Dunker Church for support. The Twentieth now found themselves uncovered in the front and on their left flank.
As the Twentieth entered the West Woods they immediately encountered heavy firing, followed by artillery barrages of grapeshot and canister. As the lines of battle were so close the Twentieth had difficulty returning fire without hitting their own men. The Confederates flanked Sumner's forces on the left, which left the Twentieth exposed to Confederate fire in front as well as on their left. The colors of the Twentieth fell four times within minutes, and Sumner ordered Colonel William Lee to withdraw. In despite of the chaos the Twentieth attempted to withdraw in good order, removing their wounded as they withdrew.
The battle continued for the entire day, with heavy fighting at the Sunken Lane and the bridge across the Antietam Creek later known as the Burnside Bridge. A late arrival by Confederate General A.P. Hill ended the battle, in which neither the Union nor the Confederates could claim a solid victory.1
1George A. Bruce, The Twentieth Regiment of Massachusetts Volunteer Infantry 1861 - 1865 (Cambridge, Massachusetts: The Riverside Press, 1906), 163-74. Richard F. Miller, Harvard's Civil War: A History of the Twentieth Massachusetts Volunteer Infantry (Lebanon, New Hampshire: University Press of New England, 2005), 169-76. James Spencer, "Record of James Spencer," Association of Officers of the Twentieth Massachusetts Regiment, Reports, Letters and Papers Appertaining to Twentieth Massachusetts Volunteer Infantry, vol. 1, p. 64-65, Twentieth Massachusetts Special Collection, Boston Public Library, Boston, Massachusetts.