This book is a systematic effort by leading international scholars to map the trends in major-power warfare and explore whether it is waxing or waning. The main point of departure is that major-power war as a historical institution is in decline. This does not mean, though, that wars between states are in general disappearing. While there is some convergence in the conclusions by individual authors, they are by no means unanimous about the trend. The articles explore different causes and correlates of the declining trend in major-power warfare, including the impact of the international structure, nuclear weapons, international law, multilateral institutions, sovereignty and value changes.