 On September 1 , the regiment , now numbering 400 , marched northwest from Centreville with the rest of Ferrero 's brigade . Although the men did not know it , they were among the handful of brigades chosen by Pope to deflect Jackson 's flanking movement . Forming battle lines in a field not far from Chantilly Plantation , the 21st watched the 51st New York advance into thick woods to probe the enemy lines . Orders were soon given to Col. Clark to lead the 21st into the woods to support the 51st . The woods were very dense and the advance was disorderly . Making matters worse , a powerful thunderstorm burst out as the 21st entered the woodland . Unknowingly , the regiment <unk> far to the left and lost contact with the 51st New York . Encountering a body of troops in their front and unable to identify them through the torrential rain , Col. Clark presumed he had found the 51st , ordered the 21st to rest in place and sent officer Lt. Colonel Joseph Parker Rice forward to make contact . The troops were , in fact , men of Jubal Early 's brigade and , before this could be ascertained , the Confederates unleashed a devastating volley on the unsuspecting 21st . " In the sudden anguish and despair of the moment , " wrote Capt. Walcott , " the whole regiment seemed to be lying bleeding on the ground . " Col. Clark immediately ordered a withdrawal and the 21st left more than 100 dead or wounded in the woods . 
