As explained in the first part of this post, espionage exceptions to primary rules of international law can only emerge under customary international law where there is evidence of state practice and opinio juris. While Part I of this post focused upon state practice, Part II zeros in on opinio juris.

Opinio Juris

Let’s accept that there is a sufficient state practice in favour of espionage operations that violate primary rules of international law. If we accept this, the immediate question is whether that practice is accompanied by opinio juris, which requires states to engage in a practice with the belief that they are acting as of right. Identifying opinio juris is problematic in the context of espionage because of the ‘policy of silence’ (Navarrete, 17) that accompanies this practice, which usually manifests as a ‘neither confirm nor deny’ attitude. In light of this, it is straightforward to conclude that state practice of espionage is not supported by assertions that it is lawful as a matter of CIL.

Interestingly, commentators argue that opinio juris in favour of the legality of espionage can be furnished by the failure of victim states to invoke international law to protest against espionage. For these scholars, silence can be interpreted as acquiescence and acquiescence can be inferred as acceptance. For example, in the context of cyber espionage Schmitt explains:

The State practice regarding exfiltration appears so thick, and the condemnation on the basis of international law so muted, that I find it implausible to argue that sovereignty is violated by these commonplace cyber operations.

The ‘silence as acceptance’ approach has merit in international law. For example, Draft Conclusion 10(3) of the International Law Commission’s Draft Conclusions on Identification of Customary International Law holds that a ‘[f]ailure to react over time to a practice may serve as evidence of acceptance as law (opinio juris), provided that States were in a position to react and the circumstances called for some reaction.’

But silence isn’t always golden. As per Draft Conclusion 10(3), silence can be only taken as acceptance where states are ‘in a position to react and the circumstances call for a reaction’. Two issues are relevant here, neither of which are adequately explored in the current literature.

First, the secrecy of state practice means that states are not ‘in a position to react’ to espionage. As such, their silence says nothing about the legality of that practice as a permissible exception to a primary rule – quite simply, it is possible that victim states are unaware that the espionage operation has occurred.

Second, if there is public state practice in favour of espionage, is silence tantamount to acceptance? The question here is when do ‘circumstances’ call for a reaction? Certainly, states that are the victim of espionage would be expected to protest and, if they do not, their silence can be regarded as acceptance of the legality of that conduct under CIL. But what about non-victim states that merely observe espionage against other states? Is this a situation calling for a reaction and, if states fail to condemn the espionage as a violation of international law, their silence can be construed as acceptance? If the activity in question violates an erga omnes obligation (which are international law obligations that have attained constitutional status and which are owed to the international community as a whole), then surely these would be circumstances calling for a reaction by non-victim states. Yet, espionage is unlikely to violate an international legal rule of erga omnes character, which typically includes those rules prohibiting genocide, torture, slavery and aggression (Barcelona Traction, para 34). Do the ‘circumstances’ that ‘call for a reaction’ go beyond the violation of erga omnes rules and, if so, what do they encompass? While this is a tricky question that requires further attention, we do not need to tackle it in this post because it suffices that, as an activity perpetrated in secret, states are not provided with the opportunity to react to espionage.

Conclusion

There must be clear and compelling evidence of state practice and opinio juris before we can admit the existence of customary exceptions to otherwise prohibitive rules of international law. Espionage is a poor candidate for a customary exception because of its inherently secretive nature. As this post reveals, the secretive nature of espionage means the two essential organs of the anatomy of CIL are absent, namely, state practice and opinio juris.