After the infamous "Mud March" of January 1863, Major General Joseph Hooker replaced Major General Ambrose Burnside as commanding general of the Army of the Potomac. In late March, Henry's 9–year old brother, Arthur, died from "the croup." After two weeks in Boston following Arthur's death, Henry returned to his regiment. On May 3, 1863, Abbott fought with his regiment during the Second Battle of Fredericksburg, Virginia, which was part of the Chancellorsville Campaign. Major General John Sedgwick, commander of the "grand division" of which the 20th Massachusetts Infantry was a part, was ordered to take Fredericksburg and then come to the aid of the bulk of the Union Army which came under heavy attack from Confederate forces at Chancellorsville. The 20th Massachusetts Volunteer Infantry Regiment was on the far right of the line and suffered only 2 dead and 13 wounded, one of whom was Oliver Wendell Holmes, Jr. who was wounded for the third time. In this battle, the Union troops overwhelmed the more lightly defended Marye's Heights. Sedgwick was stopped from joining Hooker at nearby Chancellorsville by Confederate forces at the Battle of Salem Church. Brigadier General John Gibbon's division, including the 20th Massachusetts Infantry Regiment, which had been left to hold the line at Fredericksburg, had to retreat across the Rappahannock River to join Hooker and Sedgwick, who had recrossed the river further upstream.