The phrase "act of war" is frequently used as shorthand to refer to an act that may permit a state to use force in self-defense, but more appropriately, it refers to an act that may lead to a state of ongoing hostilities or armed conflict. Contemporary international law addresses the concept of "act of war" in terms of a "threat or use of force," as that phrase is used in the United Nations (U.N.) Charter. Article 2(4) of the UN Charter provides:
|“||All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state.||”|
International legal norms, such as those found in the U.N. Charter and the law of armed conflict, which apply to the physical domains (i.e., sea, air, land, and space), also apply to the cyberspace domain.
As in the physical world, a determination of what is a "threat or use of force" in cyberspace must be made in the context in which the activity occurs, and it involves an analysis by the affected states of the effect and purpose of the actions in question. The particular attributes of cyberspace can make this determination especially difficult, including the detection of the activity, political and/or technical attribution, and a determination if the particular activity is part of a larger military operation, although these are challenges present in the "real world" as well.
Without question, some activities conducted in cyberspace could constitute a use of force, and may as well invoke a state’s inherent right to lawful self-defense. In this context, determining defensive response to even presumptively illegal acts rests with the Commander-in-Chief.