 Northern fears of a continued offensive by Robert E. Lee were realized when he launched his Maryland Campaign on September 4 , hoping to arouse pro @-@ Southern sympathy in the slave state of Maryland . McClellan 's pursuit began on September 5 . He marched toward Maryland with six of his reorganized corps , about 84 @,@ 000 men , while leaving two corps behind to defend Washington . McClellan 's reception in Frederick , Maryland , as he marched towards Lee 's army , was described by the correspondent for Harper 's Magazine : 
