Its generally accepted that Lee's incursion into Maryland was a play to garner the the support of the Southern loyalists there and gain a lot of new recruits. Had he won at Antietam he felt he would've been seen as a liberator freeing MD from Yankee oppression and that was supposed to result in waves of goodwill from the locals. As it was he dropped his battle plans and ended up leaving the battlefield non victorious. I believe the war was totally winnable right up to the third day at Gettysburg. The North was becoming war weary.
Had Lee won at Gettysburg it is interesting to speculate his next move.
The Pennsylvania capital at Harrisburg?
Washington DC?
The north was war weary. The twin victories at Gettysburg and Vicksburg reinvigorated the North, as would the fall of Atlanta in 1864.
