This book demonstrates how populist security narratives served as the driving force behind the mobilization of Republican voters and the legitimation of an 'America First' policy agenda under the Trump presidency. Going beyond existing research on both populism and security narratives, the author links insights from political psychology on collective narcissism, blame attribution and emotionalization with research in political communication on narrative and framing to explore the political and societal impact of a populist security imaginary. Drawing on a comprehensive range of sources including key interviews, campaign and policy speeches, presidential addresses, and posts on social media, it shows how progressives, political opponents, immigrants, racial justice activists, and key institutions of liberal democracy collectively became an internal Other, delegitimated as 'enemies of the people'. Developing an innovative conceptual-analytical framework of nationalist populism that expands on established concepts of political identity and ontological security, the book will appeal to students of critical security studies, critical constructivist approaches in International Relations, and US politics.