This dissertation rests on a relatively simple premise: America’s road to disunion ran west, and unless we account for the transcontinental and trans-Pacific ambitions of slaveholders, our understanding of the nation’s bloodiest conflict will remain incomplete. Whereas a number of important works have explored southern imperialism within the Atlantic Basin, surprisingly little has been written on the far western dimension of proslavery expansion. My work traces two interrelated initiatives – the southern campaign for a transcontinental railroad and the extension of a proslavery political order across the Far Southwest – in order to situate the struggle over slavery in a continental framework. Beginning in the 1840s and continuing to the eve of the Civil War, southern expansionists pushed tirelessly for a railway that would run from slave country all the way to California. What one railroad booster called “the great slavery road” promised to draw the Far West and the slaveholding South into a political and commercial embrace, while simultaneously providing the plantation economy with direct access to the Pacific trade. The failure of American expansionists to construct a transcontinental railroad during the antebellum era has discouraged close scholarly scrutiny of this political movement. Yet through their efforts, southern railroaders triggered some of the fiercest sectional struggles of the era, and carried the contest over slavery far beyond the Atlantic world. The second part of this dissertation reconstructs local political contests in Utah, New Mexico, Arizona and California to highlight the long reach of proslavery interests. Never a majority in the region, southern-born leaders wielded an outsized influence within western legislatures, courtrooms, and newspaper offices to effectively transform the Southwest into a political appendage of the slave South. With the fracturing of the Union in 1861, the project of southern expansion moved to the battlefields of a continental civil war, with several initially successful Confederate invasions of New Mexico. Even as the rebellion collapsed across the South, Confederate leaders continued to look west, authorizing yet another invasion of the region as late as the spring of 1865. The proslavery dream of a western empire almost outlived slavery itself.