This chapter analyzes the competing racial geographies of the United States and Mexico in the newly annexed territory. Mexican Americans challenged Jim Crow segregation in the Southwest through naturalization, segregation, and discrimination cases brought between 1897 and 1954. The mestizo character of plaintiffs flummoxed the racial geography of the United States in the courts, as the plantiffs' mestizaje was illegible from the perspective of a biracial organization of the American polity. Court decisions repeatedly required Mexicans to renounce their indigenous and African heritage to enjoy the privileges of whiteness, while Mexican-Americans' repeatedly claimed a whiteness that resembles mestizaje. The chapter argues against the extant Chicana/o legal scholarship that characterizes these activists as guilty of an aspirational whiteness, arguing instead, through an analysis of letters and speeches of these activists, that they challenged whiteness from within rather than suffer the psychic loss of their ancestry, preparing the way for the emergence of Aztlán in the post–civil rights era.