Historically, states have posed the greatest threats to international security, especially through wars that have caused massive death and destruction. Is that still the case? What sort of security threat does China’s growing power pose to the United States? Another fear is that of nuclear weapons “cascades,” or a “tipping point” beyond which a large number of states will acquire nuclear weapons. Is such a cascade likely? What danger would such a scenario pose to Americans? And finally, American alliances are justified on the basis of fears that current U.S. allies would engage in security competition or war with each other or with third parties in the absence of U.S. security guarantees. How likely would this result be, and what sort of threat would it pose to Americans?