"Up to this time, the South had won the major victories, but her resources, both in men and sinews of war, were diminishing and a prolonged conflict would be disastrous. It was decided that the army should invade the North via the Shenandoah and Cumberland valleys, with Harrisburg as as an objective. This route not only afforded a continuous highway, but put the army in a position to threaten Baltimore, Philadelphia and Washington from the North..."
It was on the first day at Gettysburg that the Confederates made a terrible mistake. Read about it here.
Click here to read an 1862 review about the Civil War photographs of Mathew Brady.