National security threats facing the West are fundamentally changing. Turning away from the military as an omnibus tool of aggression, hostile governments are instead frequently using tools--including subversive economics, coercion of foreign companies, gradual border violations, cyberattacks, disinformation, and arbitrary detention of foreign citizens--that are often difficult for targeted countries to immediately identify, let alone tackle. Nonmilitary aggression is easy, inexpensive, and alarmingly effective. Businesses -- American and foreign -- have already suffered significant financial losses because of gray-zone attacks. In The Defender's Dilemma, international security expert Elisabeth Brawer offers the first sustained analysis of how these tactics in the gray zone between war and peace dangerously weaken liberal democracies, which are open societies by definition and intimately connected to the rest of the world through globalization. She discusses the breadth of gray-zone aggression and presents strategies for better defense against it. These strategies involve not just governments but also civil society, a largely untapped resource.