This dissertation reconstructs the political thought of Yankton Dakota activist-intellectual Vine Deloria, Jr. (1933-2005) in order to explore how Indigenous peoples in the Americas have developed a tradition of politically engaged, anti-colonial critique—a politics of decolonization. Since World War II, democratic theorists have mounted accounts of civic inclusion and multicultural representation to both invigorate projects of democratic state- and nation-building and to respond to legacies of racial and cultural injustice. Against these accounts, I argue that settler democracies make their boundaries through colonial projects of replacement and normalized incorporation that disavow and dissolve Indigenous peoples’ separate polities. Beginning with his leadership in the National Congress of American Indians in 1964, Deloria provided a) an analysis of narratives of civic inclusion and multicultural representation as colonial and b) translated practices of decolonization emergent from the Indigenous sovereignty movement into an evolving framework of shared Indigenous concepts. The project traces Deloria’s counter-proposals through three phases: First, Deloria confidently re-theorized democratic state-building as “empire” so as to promote among Indigenous peoples an anti-colonial politics of self-determination (1964-1969). Second, Deloria aggressively reimagined Indigenous sovereignty as a distinctive variant of constituent power (1969-1975). Third, Deloria disappointedly reckoned with the durability of colonialism and capitalism as twin engines of destruction and re-described Indigenous conceptions of sacred territory, relationship, and responsibility as the ethical-political foundations of decolonization (1975-2005). Through this reconstruction of Deloria’s work in conversation with contemporary Indigenous and Settler-Colonial Studies, my project provides a basis for refashioning political theory’s core interpretive commitments to address the questions of dispossession, landlessness, self-determination, and sovereignty most apt for decolonization struggles in settler-colonial contexts.