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

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 belief 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 the belief that buttressed Manifest Destiny, and drove our foreign policy during the 19th century. 

Manifest Destiny implies that The United States, by its very nature, and because of its founding ideals, 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.

In the mid-1800s, starting in 1824 with president Andrew Jackson, and continuing on with his successors, Martin Van Buren and James Polk, American leaders used the argument of inherent moral superiority and Manifest Destiny to justify their brutal treatment of Native Americans. This treatment included policies of extermination, forced migration, ethnic cleansing and the forced relocation of tribes on reservations. James 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, and perhaps most notably from a foreign policy standpoint, 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. By the end of that war, the United States had mostly realized Jefferson and Madison's vision of a nation stretching from coast to coast.