Treacherous Foundations is the first sustained study of the theme of treachery in the founding myths of the Iberian Peninsula. It considers literary versions, in epic, chronicle and theatre, of the legends of Fernán González, Bernardo del Carpio and King Sancho II from medieval and early modern Spain and compares the representation of treachery across two critical periods in Spanish history, assessing its political, ideological, and cultural function. This book explores the role played by representations of treachery in foundational texts in highlighting the ideological tensions that arise from movements toward the creation of collective identities. It discusses in particular visions of nationhood and the monarchical state in the thirteenth and late sixteenth centuries. The theme of treachery is expanded to cover all aspects of treason and political disloyalty and, engaging with loyalty, trust and the nature of kingship, the volume sheds new light on aspects of Spanish cultural and political history, and provides insight into the nature of myth and collective memory, historical change and the collective response to crisis.