 Lee divided his forces into multiple columns , spread apart widely as he moved into Maryland and also maneuvered to capture the federal arsenal at Harpers Ferry . This was a risky move for a smaller army , but Lee was counting on his knowledge of McClellan 's temperament . He told one of his generals , " He is an able general but a very cautious one . His army is in a very demoralized and chaotic condition , and will not be prepared for offensive operations — or he will not think it so — for three or four weeks . Before that time I hope to be on the Susquehanna . " This was not a completely accurate assessment , but McClellan 's army was moving lethargically , averaging only 6 miles ( 9 @.@ 7 km ) a day . 
