 Early in September 1862 , the IX Corps , Reno commanding , returned to the Army of the Potomac . Gen. Lee , emboldened by his victories , invaded Maryland and McClellan moved the Army of the Potomac northwest from Washington to meet the Confederates . Portions of the armies clashed during the Battle of South Mountain , during which the 21st played only a small part . Psychologically , however , it was a devastating fight in that it saw the death of Maj. Gen. Reno , who had been the 21st 's brigade commander at the start of the war . " There was not a man in the 21st who did not love him , " Capt. Walcott of the 21st wrote , " he always stood with his men in battle . " 
