Tuesday, September 16, 1862
Union and Confederate armies faced each other across the Antietam. The Confederate line stretched from north to south with its flanks on the Potomac and centering on the town of Sharpsburg. Union General George McClellan prepared for battle. He ordered General Joseph Mansfield's Twelfth Corps to support General Joseph Hooker's First Corps in an early morning attack, with General Edwin "Bull" Sumner's Second Corps, including General John Sedgwick's Second Division, of which the Twentieth Massachusetts Regiment was a part, in reserve but ready to march at daybreak.1
1George A. Bruce, The Twentieth Regiment of Massachusetts Volunteer Infantry 1861 - 1865 (Cambridge, Massachusetts: The Riverside Press, 1906), 163. Richard F. Miller, Harvard's Civil War: A History of the Twentieth Massachusetts Volunteer Infantry (Lebanon, New Hampshire: University Press of New England, 2005), 168.