In the 1960s, at the height of the Cold War, US intelligence looked into the possibility of getting cats to spy on the Soviets. They considered dogs, but decided using cats would be cheaper. Oh yes, as if the expense of buying and feeding dogs would be more valuable than their ease of training. The premise of the program was that spies would not put up their guard just because a cat is in the vicinity. They might not even notice it. Cats were trained to follow directions to get to a target, and one was even implanted with audio devices and sent out on a mission. How do you think that went?
The program was dropped because researchers at the CIA learned about the futility of herding cats. Something any cat lover would have told them in the first place. -via Nag on the Lake
Further reading: Acoustic Kitty: The CIA's Spy Cat and Esther the Cold War Kitty.