Determining an act of war in the traditional domains of land, sea, and air often involves sophisticated interactions of many factors that may be outside the control of the parties involved. This monograph seeks to provide senior policymakers, decisionmakers, military leaders, and their respective staffs with essential background on this topic as well as introduce an analytical framework for them to utilize according to their needs. It develops this theme in four major sections. First, it presents the characterization of cyberspace to establish terms for broader dialogue as well as to identify unique technical challenges that the cyberspace domain may introduce into the process of distinguishing acts of war. Second, it explores assessment criteria involved with assaying cyber incidents to determine if they represent aggression and possible use of force; and if so, to what degree? Third, it looks at the policy considerations associated with applying such criteria by examining relevant U.S. strategies as well as the strategies of other key countries and international organizations, and considers how nonstate actors may affect U.S. deliberations. Fourth, it examines the influences that course of action development and implementation may have on the assessment of cyberspace incidents, such as reliable situational awareness, global and domestic environment considerations, and options and their related risks and potential consequences. It argues that the United States must also expect and accept that other nations may reasonably apply the criteria we develop to our own actions in cyberspace.