What are the limits of justified retaliation against aggression? What actions are morally permissible in preventing future aggression? Against whom may retaliation be aimed? These questions have long been part of the debate over the ethics of warfare. They all took on new meaning after terrorists hijacked four US airliners on September 11, 2001. This work considers the just aims and legitimate limits of the United States’ response to the terrorist attacks. Six essayists from the University of Maryland’s Institute for Philosophy and Public Policy pair off to discuss three ethical questions in this context. Judith Lichtenberg and Robert Fullinwider explore the moral challenges posed by terrorism. Paul Kahn and David Luban question whether modern terrorism can be addressed within existing paradigms of just war and international law. Finally, Lloyd Dumas and Benjamin Barber ask whether the US response should be military, and if not, what it might look like. As a whole, these six essays ask the fundamental question, how should the United States use its power to combat terrorism?