The theory of deterrence emerged with the advent of nuclear weapons to address the challenges of preparing for and preventing a full-scale nuclear war between the United States and the Soviet Union. The contributions to this special issue are set in a post-Cold war context, with a resurgent and aggressive Russia. The set of articles provides an outline of the theory of deterrence, the current practice of its application in deterring and, if necessary, defending by conventional forces NATO and Europe’s Eastern flank against an aggression, and critical analysis of its pertinence to cyber and hybrid warfare.