Abstract This chapter proposes a new theory of popular nationalism and war by investigating how popular nationalism factors into political leaders’ strategic decision to launch military aggression. It elucidates how popular nationalism creates two contrasting incentives with regard to the initiation of militarized conflict, and how those incentives, coupled with the two moderating variables—political elites’ perceived chance of victory in conflict and their domestic political vulnerability—ultimately shape a state’s decision to go to war or maintain the status quo. The chapter emphasizes two faces of popular nationalism: it can trigger interstate war, but it can also suppress the chance of military aggression and facilitate maintaining the status quo. The chapter closes by laying out a multimethod research design employing a survey experiment and in-depth case studies.