The philosophical underpinnings behind the notion of Manifest Destiny date back to the first Puritan settlers, such as John Winthrop, and his group’s explicit desire to build “a new Jerusalem,” or a “Shining City on a Hill.” The notion that America and its Puritan founders had a moral right and obligation to invent a better, purer world informed the notion of what came to be known as “American Exceptionalism,” which is really an extension of Manifest Destiny. Both terms imply that by its very nature, and because of its founding and ideals, the United States has a moral destiny to rule the western hemisphere and to project its power and values across the continent, by force if necessary.
Thomas Jefferson expanded on this notion when he envisioned a vast agricultural economy stretching from sea to sea. Jefferson's decision to go ahead with the Lousiana Purchase as president was a big first step in actualizing that dream. Subsequent presidents, particularly Andrew Jackson and James Polk, used the argument of inherent moral superiority and Manifest Destiny to justify their brutal treatment of Native Americans, which included policies of extermination, forced migration and ethnic cleansing. Polk was a major proponent of territorial expansion both as a military leader and as a president.
Although America’s westward expansion was rationalized and cloaked in terms of moral superiority, it was ironically fueled in large part by the rapacious appetite of Southern plantation owners and later, by ruthless industrialists, who believed it was their right and destiny to exploit the land and its untapped resources. By the 1840s, the term “Manifest Destiny” had started to be coopted by mining and railroad companies, which enjoyed the political and financial support of the federal government, and could rely on the United States Army to protect its business interests not only from Native Americans who had lived on the land, but also from laborers who had the temerity to demand better pay or safer conditions.
Finally, in 1846, President Polk led America to war against Mexico, using Manifest Destiny as a rallying cry in a war that netted the United States the territories that would become Arizona, California and New Mexico.