... decided to withdraw the troops from that point, and form a line of battle at Sharpsburg, where he would be in a position to unite with Jackson, when he should recross the Potomac at Shepherdstown. Fitz Lee , who had been with his cavalry brigade in the rear of the Federal army at Frederick, arrived at Boonsboroa during the night, and was directed by General Lee to remain there and retard as much as poss...