William S. Clark
Following his failure at Fredericksburg, Burnside was reassigned to the Department of the Ohio. The IX Corps, which he had formerly commanded, was reassigned with him. The 21st Massachusetts Infantry boarded trains for Cincinnati on March 28, 1863. They were then transported to Mount Sterling, Kentucky, to combat Confederate guerrillas. In April, due to the reduced numbers of the regiment (now about 200) several officers, being effectively without commands, chose to resign their commissions, including Col. William S. Clark. Command of the regiment passed to Lt. Col. George P. Hawkes.