Jul 1, 1863: The Battle of Gettysburg begins The largest military conflict in North American history begins this day when Union and Confederate forces collide at Gettysburg. The epic battle lasted three days and resulted in a retreat to Virginia by Robert E. Lee's Army of Northern Virginia. Two months prior to Gettysburg, Lee had dealt a stunning defeat to the Army of the Potomac at Chancellorsville. He then made plans for a Northern invasion in order to relieve pressure on war-weary Virginia and to seize the initiative from the Yankees. His army, numbering about 80,000, began moving on June...