A spy was defined by the German army staff in 1870 as one “who seeks to discover by clandestine methods, in order to favor the enemy, the position of troops, camps, etc.; on the other hand enemies who are soldiers are only to be regarded as spies if they have violated the rules of military usages, by denial or concealment of their military character.”
The Brussels Declaration of 1874 defines the conception as follows: “By a spy is to be understood he who clandestinely or by illicit pretenses enters or attempts to enter into places in the possession of the enemy with the intention of obtaining information to be brought to the knowledge of the other side.” The Hague Conference puts it in the same way.
Of the essentials of Espionage.
The emphasis in both declarations is to be laid on the idea of “secrecy” or “deception.” If regular combatants make enquiries in this fashion, for example in disguise, then they also come under the category of spies, and can lawfully be treated as such. Whether the espionage was successful or not makes no difference. The motive which has prompted the spy to accept his commission, whether noble or ignoble, is, as we have already said, indifferent; likewise, whether he has acted on his own impulse or under a commission from his own State or army. The military jurisdiction in this matter cuts across the territorial principle and that of allegiance, in that it makes no difference whether the spy is the subject of the belligerent country or of another State.
It is desirable that the heavy penalty which the spy incurs should be the subject not of mere suspicion but of actual proof of existence of the offense, by means of a trial, however summary (if the swift course of the war permits), and therefore the death penalty will not be enforced without being preceded by a judgment.
Accessories are Principals.
Participation in espionage, favoring it, harboring a spy, are equally punishable with espionage itself.