Contested Inclusion analyzes changes of official national ideologies in Mexico, Argentina, and Peru during the early and mid-20th century. During this period the three countries witnessed a transition from political to cultural conceptions of nationhood, and from elite-centered to class-based understandings of national identity and history. The extent and the timing of this change varied substantially across the three cases, with major long-term implications for political development and social policy, and subsequent struggles over national belonging. To explain the distinct transformations of nationalism in these countries I present a novel theoretical framework that calls attention to conflicts and alignments between state elites and social movements, and to the timing of state institutional development. In the exploration of my argument I draw on school textbooks to trace the contents of official national ideologies, and teacher testimonials to examine the broader reception of those discourses. The manuscript represents one of the first efforts systematically to compare different forms of nationalism in Latin America. Its argument will stir debate among historians of Mexico, Argentina, and Peru. Contested Inclusion will also be of broad interest to scholars of nationalism. The methodological approach unpacks the role of schooling in the construction of nationhood, and the general framework provides a corrective to the relative absence of theories that explain how, when, and why official national ideologies change.