Excerpts from Books and Wikipedia
"In late spring 1863, the Confederacy launched a formidable attack on the North. Robert E. Lee's army of northern Virginia defeated the invading army of the Potomac in thick forest at Chancellorsville, Virginia--but Lee lost his right-hand man, Stonewall Jackson, shot by mistake by his own sentries. Lee moved on north into western Pennsylvania, intending to seize the crucial railroad junction in Harrisburg (to disrupt Union supply lines) and perhaps go on to take Philadelphia or even Washington, DC itself. The opposing Union army was commanded by General George D. Meade. Neither army knew exactly where the other was until they bumped into each other at Gettysburg, not far from the site of the Battle of Antietam. The Confederates drove the Union soldiers out of town, but unfortunately drove them south to an excellent defensive position along the ridge." [Furtado: 1001 Days]