A mong historians, the summer of 1863 represents the traditional turning point of the Civil War. After marching into southern Pennsylvania, Robert E. Lee's Army of Northern Virginia was stopped at the Battle of Gettysburg and turned back, out of Northern territory. Two days after Gettysburg, Union general Ulysses S. Grant captured the fortress town of Vicksburg, Mississippi, after a long siege. The fall of Vicksburg allowed the federal army and navy to seize control of the entire Mississippi River, which, in President Abraham Lincoln's words, now "flowed unvexed to the sea." The loss of the Mississippi valley effectively cut the Confederacy in two—an important part of the overall war strategy decided on by Lincoln and his top commanders.

Yet a crucial military and political point had already been reached that was playing an even more decisive role in the war's outcome. This was the...