What did McPherson believe was the turning point of the war?

McPherson doesn't believe that the Civil War was decided by a single battle but by a series of sustained and costly operations, which was one of the main factors that made the conflict the first modern war. That said, McPherson argues that the Battle of Antietam was pivotal in that it led to what he regards as one of many turning points in the war: Lincoln's decision to issue the Emancipation Proclamation.
Though not quite a decisive victory for the North, Antietam was nonetheless enough of a victory to change the whole nature of the war. From then on, the war to save the union was linked to the abolition of slavery. General Lee's failure to capture the Union slave state of Maryland dealt a final blow to any lingering hopes of a compromise settlement, ensuring that the remainder of the conflict would be conducted by Lincoln as more of a moral crusade than as a war to keep the nation together.