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.
The passage discusses the reasons behind Lee's lack of strategic offensives following Gettysburg and the speculative viewpoint of Lost Cause writers regarding a potential Confederate victory leading to the war's conclusion.