The correct answer to this open question in the following.
Robert Lee was hoping to accomplish two things when he invaded the North a second time: to capture the Capital of the Union (Washington D.C.), moving the battle beyond the Potomac, and to get more supplies for the Confederate army as well as the support of European nations.
Robert E. Lee's Army crossed Maryland, heading to Washington in September 1872, after a great victory in the Second Battle of Manassas. However, his aspirations ended when the Union Army defeated the Confederates in the Battle of Antietam.