In the last few weeks, I have been receiving a lot of inquiries about peacetime spying and international law. In part, that is because of some of my blog entries on this platform and in part it stems from an article I published several years ago on this topic.
At the time of that publication, I recall one of the peer reviewers expressing puzzlement about why I would focus on the theoretical question of peacetime spying and international law. After all, the spies didn't worry too much about international law. Part of the reason for that may be (as I have argued in some of my recent postings) the creative ambiguity around peacetime spying and international law.
Indeed, there is an academic literature (summarized in the article) urging that because all states spy on each other, there is some sort of customary international law exception to spying. From a doctrinal perspective, I think this literature is incorrect. State practice alone does not make something customary international law. State practice with a sense that the practice is legally obligatory or at least permissible (that is, opinio juris) can. But states regularly condemn spying conducted against them. And when they spy, they do so covertly -- in part for effectiveness reasons and but also in part to avoid condemnation and embarrassment.
Under these circumstances, it is very difficult to argue that the spying is done with a sense of opinio juris. For this reason, I do not agree that there is a customary international law exception that allows spying, one that sweeps away sovereignty concerns.
Which brings me to the Federal Court, and Justice Blanchard's 2008 judgment in Canadian Security Intelligence Service Act (Re). At issue was whether the Canadian Security Intelligence Agency could obtain a warrant for overseas surveillance (so, spying). The court said no, and in so doing raised international law issues (and specifically, sovereignty). In my article, I suggest that because of the debate (in at least the academic literature) over peacetime spying, the court was probably a bit more definitive on this issue than was really warranted. But I believe its instincts were almost certainly correct. Here are the key paragraphs:
 The intrusive activities that are contemplated in the warrant sought are activities that clearly impinge upon the above-stated principles of territorial sovereign equality and non-intervention. Further, the activities are likely to violate the laws of the jurisdiction where the investigative activities are to occur. This is not disputed by the Service. The amicus maintains that there is no evidence which would allow the Court to make such a determination. In my view, to require such evidence to be adduced would be to place a heavy burden on the Service. The Service intends to execute the warrant wherever the targets are located. Understandably, no specific foreign state is identified in the application since the Service is likely unable to predict where these targets may travel once they leave Canada. It is therefore difficult, if not impossible, to lead evidence as to the legality of the investigative activities sought to be authorized in a given jurisdiction at the application stage, since no foreign state is identified.
 Among the powers sought to be authorized under the warrant are: the ability to obtain access, install any thing [portion deleted by order of the Court]; search for, examine, take extracts from, make copies of, or otherwise record information. Given the intrusive nature of the activities at issue, it is reasonable to infer that the activities are likely to violate the laws of the jurisdiction(s) where the warrant is to be executed. In any event, absent consent of the foreign state, the investigative activities at issue impinge upon the territorial sovereignty of the foreign state.
 By authorizing such activities, the warrant would therefore be authorizing activities that are inconsistent with and likely to breach the binding customary principles of territorial sovereign equality and non-intervention, by the comity of nations. ...
As best I know, this is the only court decision in a "five eyes" state that opines so clearly on international law and extraterritorial spying.
Fast-forward five years, and these judicial pronouncements look very apropos the (as of yet quite muted) discussions on the international legal issues raised by "five eyes" spying.
But of course, I am still being an academic. There is no way that international law will supplant politics and diplomacy in redressing the recent controversies.