The Army of the Potomac, under its new commander George Meade, raced to intercept Lee's Army of Northern Virginia as it invaded Pennsylvania. The two armies met at Gettysburg in Adams County and over three days fought the largest engagement of the Civil War. That Meade held the best defensive positions was critical and the Confederate Army was defeated and forced to withdraw back across the Potomac. The pivotal battle also produced the largest toll of the war, with the Union suffering 23,000 casualties and the Confederates around 28,000. (By John Osborne)
James M. McPherson, Ordeal by Fire: The Civil War and Reconstruction, 3rd ed. (New York: McGraw-Hill, 2001), 352-356.