What is the summary of "Long Remember: The Summer of '63," chapter 21 from James McPherson's book Battle Cry of Freedom?

In this chapter, McPherson narrates the events of the summer of 1863, a period which marked two significant turning points of the Civil War. The chapter begins with an account of the capture of Vicksburg, Mississippi, the so-called "Gibraltar of the West" that was the key to controlling the Mississippi River. After failing to capture the city by the spring of 1863, Union forces under Ulysses S. Grant slowly tightened their grip by sending gunboats past Confederate batteries on the Mississippi River to surround the city. Subsequent attacks, though bloody, cut off the city from the outside, and beginning in May of 1863, Grant and the Union armies dug trenches and settled into a siege. After attempts to relieve the beleaguered Confederate forces (and civilians) failed, Confederate General John Pemberton surrendered on July 4, 1863. This was almost the same time, as McPherson points out, that Robert E. Lee's attempts to smash the Union army at Gettysburg failed, and the rest of the chapter concerns that extraordinary battle and the events that led up to it. McPherson begins by detailing the Confederate victory at Chancellorsville, Virginia, an astonishing defeat of the Union that came at a heavy cost for Lee's Army of Northern Virginia: among the 13,000 Confederate casualties was General Thomas "Stonewall" Jackson. With this victory in hand, Lee went on the offensive, invading western Pennsylvania in a bold gambit in June of 1863. He hoped, as McPherson writes, to gain a smashing victory on Northern soil and embolden so-called "Peace Democrats" in the North that sought to bring the war to a negotiated end. After marching through the Pennsylvania countryside, his force stumbled into elements of the Union Army at Gettysburg. A three-day battle ensued in which the Confederate forces failed, after multiple attempts, to shatter the Union lines and drive them from the field. This bold strategy thus backfired. Lee was forced to retreat into Virginia. As McPherson concludes at the end of the chapter,

Though the war was destined to continue for almost two more bloody years, Gettysburg and Vicksburg proved to have been its turning point.