The last decade has witnessed the heightened destructive potential of cyber attacks; correspondingly, cyberspace has become the new battlefield for nation-states in conflict. Yet jus ad bellum-the body of international law governing legitimate use of force-provides little guidance about the legality of a cyber attack or when such an attack becomes an act of war justifying resort to responsive force. This Comment provides a new analytical framework for addressing that question. It begins by clarifying definitional ambiguities in the literature on cyber attack, setting forth a definition that focuses on computer networks as the instruments, rather than objects, of attack. It examines the technical concepts and considerations that influence the use-of-force analysis and animate the evaluation of potential analytical frameworks. It then discusses the governing jus ad bellum standards and critiques the leading approaches to assessing cyber attacks under these standards. This Comment departs from the traditional and accepted models of inquiry, drawing on cyber security research to propose a framework centered on cyber-physical systems that addresses cyber attacks under the laws of just war.