Spy, in war, one employed to penetrate the enemy's lines and ascertain his condition and plans. Spies have always been employed in warlike operations, and writers on the laws of war lay down the principles which are to regulate their conduct. Though thus recognized, a stigma is attached to their employment as one to which falsehood and treachery are indispensable, and a captured spy is not admitted to the privileges of a prisoner of war, but is put to an ignominious death. The difficulty of determining who is to be treated as a spy is sometimes very great. Presumptively one who is within the enemy's lines in disguise or wearing the enemy's uniform is a spy; but the dress is only a circumstance indicative of an intent to deceive, and other circumstances might be equally conclusive. One belligerent may employ the subjects of the other as spies, but a peculiar infamy attaches to one who to a discreditable occupation adds the atrocious crime of treason. A spy is not restricted to obtaining information; he may inflict injury upon the enemy, so that he do not resort to assassination, poisoning, or other means which, in the words of Vattel, "affect the common safety of human society." Inciting a spy to such atrocity would subject a commander and his forces to retaliation.

An employment at once so dangerous and so discreditable cannot be forced upon any one; the commander must usually procure his spies by heavy rewards.