

Recent studies, in particular, have started to question the compatibility of surveillance practices with human rights law. That being said, more critical writings also exist. This often resulted in a literature that defended the legality of intelligence-gathering activities as conducted by US agencies.

American authors, most of whom held public office, dominated the field. The analyses provided then focused on general international law and on the principle of state sovereignty. It was only in the context of the Cold War-in light of various incidents that opposed the United States and the USSR-that authors started to take a real interest in interstate spying activities on land and sea, and in air and space. Moreover, until quite recently, the legal issues raised by espionage have largely been ignored by the doctrine. Only the status of spies in times of war is, in fact, subject to specific international rules. Although such operations are a very old and common practice, it is paradoxically not regulated by a single and coherent legal regime under international law. It should also be noted that, against common assumptions, the notion exclusively refers to the gathering of information and does not refer to covert operations carried out by secret services more generally. While for a long time espionage essentially took place in the physical space, in the 21st century, it primarily takes place in the cyberspace. This classical conception has evolved to also include surveillance programs implemented by intelligence agencies toward individuals as well as company-to-company industrial espionage. International espionage consists of the access, on behalf of a state, to information that is held by another state and considered as confidential or strategic, in the military, security, or economic field.
