By the summer of 1863, Robert E. Lee thought of his Army of Northern Virginia as nearly unstoppable. It had survived McClellan and his much vaunted Peninsula Campaign and had defeated the Army of the Potomac at Manassas, Fredericksburg, and Chancellorsville. The Union army, on the other hand, had been demoralized by constant defeats and by commanders who hesitated to fight and take risks. After two years, Lee's home state of Virginia lay in waste. Out west, Ulysses S. Grant laid siege to Confederate-held Vicksburg. Its capture would result in control of the Mississippi River and the split of the South. Lee devised a daring plan where he would march his men into Pennsylvania and threaten Philadelphia and Baltimore. In doing so, he hoped to draw the fighting away from Vicksburg and Virginia, giving it some relief from the war. Much of Lee's confidence was on his generals, James...