 As the Union army retreated towards Washington , it paused to re @-@ group at Centreville , Virginia . On August 31 , 1862 , Gen. Lee ordered Lt. Gen. Jackson to flank the Union right and to cut the Union army off from Washington . This flanking effort resulted in the Battle of Chantilly which would prove to be the most disastrous engagement of the entire war for the 21st Massachusetts . 
