This invaluable text assesses the current research on the causes of both war and peace. In this revised third edition—now with a brand new chapter on the Russian-Ukraine War—leading international relations scholars explore the role of territorial disputes, power, alliances, arms races, rivalry, and nuclear weapons in bringing about war; the outcomes and consequences of war; and the factors that promote peace, including democracy, norms, capitalist economies, and stable borders. The revised third edition includes a section on emerging trends in research on cyber war, the environment and climate change, leaders, war financing, and trends in interstate conflict. Reviewing fifty years of scientific research, the contributors provide an accessible and up-to-date overview of current knowledge and a road map for future research.