What was Manifest Destiny and what impact did it have on the United States in the mid-1800s?

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Manifest Destiny was an ideology, especially prominent in the 1840s and 50s, that revolved around the notion that the United States was destined (i.e. divinely chosen) to expand its borders westward to the Pacific Ocean. This ideology, as historians have emphasized, was based on American faith in its own superiority, a belief that had three different facets. First, Americans thought their democratic institutions made them superior to other peoples, who they believed despotic. Second, white Americans thought themselves superior to the native peoples and Mexico, the nation that controlled most of the territory Americans sought. Finally, Protestant Americans believed that Roman Catholicism (the religion of most Mexican peoples) was antithetical to democratic institutions. The ideology of manifest destiny impacted American politics in important ways. In short, it helped cause the Mexican War, as Americans wished first to annex Texas (which declared its independence from Mexico in 1836) and then California and the rest of the Southwest. It also justified the taking of Native lands in the region, though this was hardly a novel development for Americans. Manifest Destiny, associated with the Democratic Party was the main issue in the election of President James K. Polk in 1844. Polk advocated both the annexation of Texas (which, it was commonly understood, would likely lead to war with Mexico) and the assertion of American claims on territory in Oregon. Under Polk's leadership, the United States went to war against Mexico, and under the terms of the treaty that ended that conflict, the nation received a vast swath of territory encompassing California and the entire Southwest.

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