The book itself has both Confederate and Union efforts at spying.......but the title in part gives something away..............1st look back.............before the US civil war.........at just military intel..............the Americans used spies in the revolutionary war...............the Mexican American war............etc................but after the wars............such departments.......small probably.....if they were even departments really..........pretty much disappeared.......ok, look now........in 2015.......the American CIA.......by itself, has almost taken over planet earth..........so a book......entitled......the Unions Secret War.......means Federal........and the north won the war...............thus a continued effect......

American Civil War spies

From Wikipedia, the free encyclopedia

Tactical or battlefield intelligence became very vital to both armies in the field during the American Civil War. Units of spies and scouts reported directly to the commanders of armies in the field. They provided details on troop movements and strengths. The distinction between spies and scouts was one that had life or death consequences. If a suspect was seized while in disguise and not in his army's uniform, the sentence was often to be hanged. A spy named Will Talbot, a member of the 35th Battalion, Virginia Cavalry, was left behind in Gettysburg after his battalion had passed through the borough on June 26–27, 1863. He was captured, taken to Emmitsburg, Maryland, and executed on orders of Brig. Gen. John Buford.^[1]