Manifest Destiny was the belief that the United States was destined to expand from the Atlantic seaboard to the Pacific Ocean; it has also been used to advocate for or justify other territorial acquisitions. Advocates of Manifest Destiny believed that expansion was not only good, but that it was obvious ("manifest") and certain ("destiny"). Originally a political catch phrase of the 19th century, "Manifest Destiny" eventually became a standard historical term, often used as a synonym for the expansion of the United States across the North American continent.

The term was first used primarily by Jacksonian Democrats in the 1840s to promote the annexation of much of what is now the Western United States (the Oregon Territory, the Texas Annexation, and the Mexican Cession). It was revived in the 1890s, this time with Republican supporters, as a theoretical justification for U.S. expansion outside of North America. The term fell out of usage by U.S. policy makers early in the 20th century, but some commentators believe that aspects of Manifest Destiny, particularly the belief in an American "mission" to promote and defend democracy throughout the world, continues to have an influence on American political ideology.[1]

This article is not a history of the territorial expansion of the United States, nor is it the story of the westward migration of settlers to the American frontier. Manifest Destiny was an explanation or justification for that expansion and westward movement, or, in some interpretations, an ideology or doctrine which helped to promote the process. This article is a history of Manifest Destiny as an idea, and the influence of that idea upon American expansion.