 The next day , McClellan began his attack with several advances by the Union right flank , far from the position of the 21st . For much of the day , the IX Corps could hear the fighting and awaited orders to take the bridge in their front . The 21st even was fortunate enough to have a mail call that morning , and the men sat down to read letters from home while fighting raged around the Dunker Church two miles ( 3 km ) away . Finally , orders came at noon and Ferrero 's brigade was moved closer to the bridge . It was a difficult target . On the opposite bank , a wooded bluff rose 100 feet ( 30 m ) above the creek . Concealed there was a small force of 400 Confederates who would successfully hold thousands of Union troops at bay for hours . The men of the 21st watched several successive charges on the bridge by brigades of the IX Corps , all of which were repulsed . Finally , Ferrero 's brigade was ordered forward . Reaching the bridge at a run , the 21st was ordered to take up a position next to the left abutment , supplying covering fire , while the 51st New York Infantry and the 51st Pennsylvania Infantry finally carried the position . As the Confederates retreated , Ferrero 's brigade was soon stationed at the top of the bluff . Later in the afternoon , around sunset , a Confederate counterattack nearly drove the IX Corps back across Antietam Creek . Ferrero 's brigade held the bluff , however , playing a key role in checking the counterattack , and taking severe casualties in the process . The 21st listed 10 killed and 35 wounded , casualties of 30 percent . 
