 After resting on their arms through the night , the 21st awoke to light fighting in their front . Over the course of the morning , Union forces gradually shifted to the left , including the 21st , and it became apparent that the Union right flank had collapsed . Meanwhile , Lee 's forces had joined Jackson 's and were pressing the Union left flank with a massive and rapid assault . Pope 's entire army was soon in retreat . Ferrero 's brigade was moved by Brig. Gen. Reno to a position on Henry House Hill to cover the Union retreat along the Centreville Turnpike . The brigade , with the 21st in the center , successfully held off Confederate advances from 7 to 9 pm , buying critical time for the Union army . Finally , under the cover of darkness , the brigade quietly retired across Bull Run . During this defeat for the Union army , the 21st suffered light casualties of seven wounded , or two percent . 
