Scott D. Sagan is professor of political science at Stanford University and codirector of the Center for International Security and Cooperation. He is the author of The Limits of Safety: Organizations, Accidents, and Nuclear Weapons and Moving Targets: Nuclear Strategy and National Security. Kenneth N. Waltz is Emeritus Ford Professor of Political Science at the University of California at Berkeley and senior research associate at Columbia University’s Saltzman Institute of War and Peace Studies. His books include Man, the State, and War: A Theoretical Analysis and Theory of International Politics.