The uproar in Europe over spying by the U.S. National Security Agency has led to calls for a treaty or code of conduct to limit espionage. To understand why this is naive, imagine a treaty to ban sex. It would be honored in the breach. States, too, have an overwhelming natural impulse: to spy.
Spying is (or was until Edward Snowden) largely covert; no one freely admits to doing it. It is also one of the last preserves of the absolute sovereign, unconstrained by law — think Louis XIV in a trench coat.
This is one reason why there are international laws for trade, travel and warfare, but almost none governing espionage, except the one that allows spies to be shot if captured in civilian clothes.