While knowledgeable observers rightly discounted the geopolitical significance of China’s launch of the refurbished Russian aircraft carrier Varyag last August, the event did underscore the salience of the topic of this H-Diplo roundtable. No student of international relations can be indifferent to the questions that Michael Horowitz addresses in Diffusion of Military Power. Will China ultimately develop true carrier warfare capacity? Why has U.S. supremacy in this area gone unchallenged for over half a century? How likely is it that China will emulate other aspects of U.S. military power? How many new nuclear powers are we likely to see? Will Iranian drones soon patrol American skies? Horowitz develops and tests a bold structural- and material interest-based theory to explain the propensity of military innovations to diffuse through the international system.