How will continued proliferation of nuclear weapons change the global political order? This collection of essays comes to conclusions at odds with the conventional wisdom. Stephen Rosen and Barry Posen explore how nuclear proliferation may affect US incentives to confront regional aggression. Stephen Walt argues that regional allies will likely prove willing to stand with a strong and ready United States against nuclear-backed aggression. George Quester and Brad Roberts examine long-term strategic objectives in responding to nuclear attack by a regional aggressor. Richard Betts highlights the potential for disastrous mistakes in moving toward and living in a world heavily populated with nuclear-armed states. Scott Sagan explains how the nuclear nonproliferation policies best suited to some states can spur proliferation by others. Caroline Ziemke shows how the analysis of a state's strategic personality can provide insights into why it might want nuclear weapons and how its policies may develop once it gets them. And, Victor Utgoff concludes that the United States seems more likely to intervene against regional aggression when the aggressor has nuclear weapons than when it does not.