WASHINGTON—The National Archives on Thursday unveiled a handwritten note by Abraham Lincoln exhorting his generals to pursue Robert E. Lee's army after the battle of Gettysburg, underscoring one of the great missed opportunities for an early end to the Civil War. An archives Civil War specialist discovered the July 7, 1863, note three weeks ago in a batch of military papers stored among the billions of pages of historical documents at the mammoth building on Pennsylvania Avenue. The text of Lincoln's note has been publicly known because the general to whom Lincoln addressed it telegraphed the contents verbatim to the...