I doubt if any conflict in American history used more spies and undercover agents than the Civil War. Speaking the same language, and members of the same society, it was easy for both sides to use agents freely. Agents from the border states were especially prized as they could easily pass for either Union or Confederate sympathizers from their state of origin. A typical use of an agent was outlined in the following letter written 150 years ago:
Wheeling, Va., October 21, [1862.]
Maj. P. Zinn, Commanding Camp Chase.
Sir: The bearer, Washington Cline, by special direction of Governor Peirpoint is sent to Camp Chase, Ohio, apparently as a prisoner but in reality on secret service. He is to be committed as usual, but in such a way as to enable him to have full access to all the prisoners sent from Marion County, Va., whom he may desire to see. It is hoped through the agency of this man that several cases of horse-stealing, depredations on private property, &c., will be brought to light. The object is for him to gain the confidence of these men who are prisoners from Marion County, Va., and now at Camp Chase, get letters from them to their friends in their section of country, and operate after his release for the conviction of those who have offended against the laws and so far avoided punishment. You can have such an understanding with the bearer and others that it shall be represented he is arrested for refusing to take the oath of allegiance. Whenever his plans are matured he should notify you of his willingness to take the oath of allegiance, which can then be administered to him and transportation given him to Wheeling to report to the Governor for further orders.
By assisting to carry out this plan you will confer a great favor upon the State authorities here. Should it be necessary to refer this matter to any one else to facilitate its execution or approve its design, please advise me as soon as possible.
Jos. Darr, Jr., Major and Provost-Marshal-General.
Such spies and undercover agents took great risks. Both sides meted out swift punishment to spies and undercover agents, usually involving a noose after the briefest of courtmartials.