Thursday, June 26, 1862
Confederate General Robert E. Lee took the initiative and the offensive against the Union army for the next six of the Seven Days Battles. Lee's plan at Mechanicsville was to attack the Union Fifth Corps, the right wing of the Union line under the command of General Fitz-John Porter. Lee felt that Porter's Fifth Corps was vulnerable to attack, as it was the only Union corps north of the Chickahominy River.
Lee ordered General Thomas "Stonewall" Jackson's division to strike the Union line first, followed in sequence by divisions led by Generals Ambrose Powell Hill, Daniel Harvey Hill, and James Longstreet. Lee's battle plans were not followed, as Jackson's men, fatigued by their long march from the Shenandoah Valley, arrived late to the battle. Due to the delay A.P. Hill began his offensive without orders, and General Porter was able to reinforce his line and repulse repeated Confederate attacks. Overall the Battle of Mechanicsville (also known as the Battle of Beaver Dam Creek) was a tactical Union victory.
The Twentieth Massachusetts Regiment was not engaged at Mechanicsville, and remained in position behind fortifications near Fair Oaks.1
1George A. Bruce, The Twentieth Regiment of Massachusetts Volunteer Infantry 1861 - 1865 (Cambridge, Massachusetts: The Riverside Press, 1906), 104-8. Richard F. Miller, Harvard's Civil War: A History of the Twentieth Massachusetts Volunteer Infantry (Lebanon, New Hampshire: University Press of New England, 2005), 136-37.