In 2009, the NATO Cooperative Cyber Defence Centre of Excellence (CCD COE) in Tallinn, Estonia, established an “International Group of Experts” to conduct the first comprehensive examination of the international law governing cyber warfare. The group consisted of twenty international law scholars and practitioners, including senior military officers responsible for legal advice on cyber operations. Three organizations provided observers to the process: the International Committee of the Red Cross, NATO's Allied Command Transformation, and the United States Cyber Command. A team of technical experts provided advice throughout the process. The resulting product of the three-year process was the Tallinn Manual on the International Law Applicable to Cyber Warfare.

The authority of the International Group of Experts is not to be exaggerated. All members participated in their personal capacity. Moreover, no attempt was made to ensure geographical representation. Instead, participants were selected based on their mastery of the relevant law or their sensitivity to the cyber contexts in which that law would be applied, or both. Although a number of States took the opportunity to informally provide feedback on drafts of the Tallinn Manual, this was always done in an unof?cial, non-attributable, and non-binding manner. Other international experts served as peer reviewers of the product, providing advice on revisions and corrections. Ultimately, all members of the International Group of Experts agreed with the formulation of the so-called “Rules” set forth in the Manual. They also agreed that the accompanying Commentary fairly explained how each rule was meant to be interpreted and applied, and fully captured any differences of opinion in that regard. Ultimately, the Tallinn Manual should be characterized as a consensus academic work by an international group of experts who devoted three years to identifying the lex lata applicable to cyber warfare.