Between 1845 and 1848, the United States doubled the size of its land holdings in North America, as Texas, Oregon, California, New Mexico, and other western regions were placed under the umbrella of U.S. sovereignty. Echoing John L. O’Sullivan’s famous phrase, historians have deemed these acquisitions “Manifest Destiny,” and have assumed that U.S. expansion – whether for good or ill – was foreordained. Yet this understanding fundamentally fails to take into account the history of the decade prior to 1846, when Americans throughout the continent believed that it was more likely that the United States would not expand beyond its borders. Examining five groups of Americans operating at the nations geographic and/or social margins, this dissertation argues that these groups hoped to achieve sovereignty outside of the United States. Nurtured by Jacksonian rhetoric that celebrated local government and personal ambition, and wary of – and at times running from – a United States mired in depression and uncertainty, these Americans were, in effect, forming their own “breakaway republics.” To validate their goal of self-sovereignty, breakaway republicans looked to the independent Republic of Texas, often referring to Texas to explain their objectives, or looking to Texas as an ally in achieving them. Between 1836 and 1845 – what this dissertation defines as “the Texas Moment” – Texas’ independent existence presupposed a different map of North America, where peoples of the northern, southern, and western borderlands carved out polities for themselves. With Texas in mind, even Americans who did not share the goals of breakaway republicans believed that independent American-led polities on the continent were likely, acceptable, and perhaps even desired. However, to a cabal of Democratic expansionists and James K. Polk in particular, this future was unacceptable. After winning the presidency after an unlikely series of contingencies in 1844, Polk and his allies laid the groundwork for a dramatic expansion of the U.S. state – and thereby a dramatic expansion of U.S. territory. Their actions ended the Texas Moment, thereby subsuming the actions of breakaway republicans and hiding their collective existence from later historians. Ultimately, the events of the mid-1840s were hardly the logical culmination of America’s expansionist destiny, but a profound rupture of the status quo. / History