Following the U.S. annexation of a vast swath of northern Mexico in 1848, a range of English- and Spanish-language authors who lived in the region composed fictions narrating the transformations of government and sovereignty unfolding around them. Contributors to this body of writing include both long-canonized and recently recovered authors from the U.S. and Mexico: John Rollin Ridge, Mark Twain, María Amparo Ruiz de Burton, Frank Norris, Heriberto Frías, Lauro Aguirre, Teresa Urrea, and others. “The Borderlands Aesthetic” reconstructs this transnational literary history in order to create a revised account of the aesthetics and politics of realist narrative. The realism of these novels and narratives lies in their presentation of changing social and political landscapes in the nineteenth-century borderlands: less concerned with individual psychology than with social relations and institutions, the works I study construct verisimilar and historically specific milieus in which characters experience the incorporation of border regions into the U.S. and Mexican nation-states. My chapters show how these novelistic worlds archive fugitive histories of competing sovereignty claims, porous borders, non-state polities, and bureaucratized dispossessions. My research thus presents a more extended literary history of novelistic narrative in the borderlands than is commonly recognized: while the borderlands novel is often treated as a form of twentieth-century fiction concerned especially with cultural hybridity, I locate the genre’s emergence a century earlier in writing more concerned with institutions than identities. Early borderlands narratives construct the institutional milieus of annexation and its aftermath using discontinuous and interruptive formal structures: jumps between first- and third-person narration, plots that wander away from conclusions, juxtapositions of discrepant temporalities, and shifting levels of fictionality. These persistent aesthetic breaks can seem at odds with conventional realist aesthetics. By the second half of the nineteenth century, proponents of realism like William Dean Howells valued the mode not only for its provision of verisimilar details but also for how it embedded characters in organic and cohesive social wholes via continuously thick description and interconnected plots. Yet I argue that it is the turn away from such narrative techniques that serves as an engine of realism in the borderlands: with their aesthetic breaks and interruptions, these works construct a fabric of social and political relations that is not a single totality but a multi-layered and division-marked assemblage. I contend that the interruptive structures of borderlands narratives are not manifestations of an alternate formation of realism but distillations of an underappreciated tendency within the mode more generally to dramatize social division via formal discontinuity. That tendency is especially apparent in the works I study because the massive social upheaval following the political reorganization of the North American southwest prompted particularly pronounced aesthetic ruptures in borderlands novels and narratives. What the aesthetic breaks of this body of writing make perceptible are varied histories of political institutions beyond the sovereign nation-state, from the flexible male homosocial networks of Silver Rush miners to the railroad monopolies ruling Gilded Age California. These histories are occluded in other forms of social representation—like censuses, travelogues, and police surveillance networks—that construct territories and populations as stable and readily knowable social wholes. This literary archive thus challenges the trend in contemporary scholarship to accuse nineteenth-century realism of reproducing the perspectives and values of dominant institutions; I contend that these borderlands narratives make sensible precisely the institutional arrangements that destabilize U.S. and Mexican state efforts to secure sovereignty. My research thus identifies a new model of novelistic politics: by making sensible the limits of the nation-state’s hold on power and the range of polities existing alongside its institutions, borderlands narratives lay bare the contingency of existing social hierarchies and invite readers to contest them. My chapters develop these lines of argument by analyzing the forms of aesthetic break that circulate through borderlands literary networks at key moments in the region’s history of governance. Chapter one shows how Joaquín Murrieta novels written in English, French, and Spanish feature interpolated scenes of theatrical address that reveal the precariousness of U.S. power in the southwest just following the U.S.-Mexico War. Chapter two focuses on Mark Twain’s writing, especially Roughing It (1872), arguing that his use of digressive narration serves as an effective technique for representing the social institutions and relations of U.S. American and Chinese populations of the Gold and Silver Rush eras. Chapter three argues that the anti-railroad novels of María Amparo Ruiz de Burton and Frank Norris employ discrepant narrative temporalities to diagnose the techniques quasi-sovereign railroad companies use to rule borderlands populations. Chapter four examines how narratives by Heriberto Frías, Lauro Aguirre, and Teresa Urrea use swings between fiction and non-fiction to bear witness to state violence in the northern Mexico town of Tomóchic. A conclusion reflects on the literary life of the 1915 “Plan de San Diego” in novels by Sutton Griggs and Américo Paredes in order to suggest an endpoint for the study. By demonstrating how formal breaks serve as realist narrative techniques in borderlands fiction of the nineteenth century, my dissertation shows this body of writing to constitute a crucial chapter in the history of the Euro-American novel.