 Following these failed assaults , both armies settled into trench warfare around Petersburg . There were numerous attempts , during the summer of 1864 , on the part of the Union army , to dislodge the Confederates from their lines . One of the most disastrous for the Union army was the Battle of the Crater during which explosives were detonated in a large mine tunneled beneath the Confederate entrenchments , temporarily creating a gap in their lines . Elements of the IX Corps , under the command of Maj. Gen. Burnside , were to charge through the gap , but instead became trapped in the enormous crater . Brig. Gen. James Ledlie 's division , of which the 21st was a part , led the attack . The division had been selected at the last minute over the United States Colored Troops who had actually trained for the operation . Unprepared , Ledlie 's division delayed in their attack , allowing the Confederates time to regroup . The 21st managed to advance , mingled with other regiments , to the furthest point penetrated by Union troops but by the afternoon they made the retreat " pell mell " with the rest of Burnside 's IX Corps . 
