Does nationalism lead to interstate war? This book challenges the existing presumption about the link between nationalism and war and systematically investigates how popular nationalism affects a country's decision to launch military aggression. In doing so, the book makes a provocative and novel claim that popular nationalism has not only a conflict-inducing effect but also a restraining effect and identifies the conditions under which popular nationalism causes war. Specifically, the book claims that popular nationalism leads to war only when leaders who confront it are very confident about their chance of achieving complete victory in conflict or they are politically vulnerable. If these two conditions are not met, popular nationalism has a restraining effect, making leaders seek the status quo and avoid the use of force. The book first shows the restraining effect of popular nationalism focusing on China through a survey experiment and an in-depth case study on the territorial dispute between China and Japan in the East China Sea. It then offers a comprehensive historical and contemporary analysis of when popular nationalism's restraining effect turns into a conflict-inducing one through case studies on the War of 1812 and the Falklands War. The book provides important insights into whether popular nationalism could put great powers like the United States and China on a collision course and offers broad policy implications for how we can prevent war driven by popular nationalism.