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The United States embarked on a dramatic expansion of territory after the end of the American Revolution and into the early nineteenth century. Though the nation's boundaries consistently changed over a period of roughly seventy years, the country experienced its largest accumulation of land during the 1840s, acquiring Texas, Oregon, California, and New Mexico in only a few years. During this decade, Americans came to believe in "Manifest Destiny," which held that American settlers were predestined to conquer the continent. Manifest Destiny was not the only reason for American Expansionism, however. American leaders also sought material gains from the land they set out to occupy. Expansion efforts were also undertaken in the spirit of imperialism as the United States became more anxious to claim its status as a world power alongside Great Britain, France, and Spain. In several decades, the United States procured more than two million square miles of territory in North America. Expansionism was not limited to the North American continent, however; many seafarers extended their reach across the Pacific Ocean in pursuit of Chinese trading profits.

Asserting the “clear and unquestionable” title of the United States to all of the Oregon country, a vast region occupied jointly by the United States and Great Britain, the Democratic platform of 1844 pressed for what it called the “reoccupation” of Oregon. It thus placed the party squarely in favor of what an 1845 magazine article was to call the nation's “manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions.” The phrase “manifest destiny,” written in connection with the annexation of Texas, was applied to the Oregon dispute in an influential newspaper editorial and in the halls of Congress. It soon swept the entire nation.