 As Gen. Robert E. Lee advanced on Pope 's position along the Rapidan River , Pope quickly withdrew his army north . The brigade to which the 21st belonged , now commanded by Col. Edward Ferrero , frequently found itself in the position of rear guard during this movement , skirmishing with the advancing Confederate cavalry . By the time the 21st reached Warrenton , Virginia , on August 27 , it was clear to the men that something was wrong . Pope was seen riding by the regiment , " looking warm and excited " , and the men learned that Lt. Gen. Thomas Jackson had flanked the Union army and was now in their rear . 
