This is based on the observation that, after Gettysburg, Lee's army conducted no more strategic offensives—his army merely reacted to the initiative of Ulysses S. Grant in 1864 and 1865—and by the speculative viewpoint of the Lost Cause writers that a Confederate victory at Gettysburg might have resulted in the end of the war.
This observation stems from the idea that Lee's army stopped offensive operations after Gettysburg, and that some historians believe a Confederate victory at Gettysburg might have led to an end to the Civil War.