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|Ideas and policies|
Manifest destiny is (or was, due to its far greater prevalence in the 19th century) the idea that the United States had the inherent right to expand all the way to the Pacific Ocean. The hyperbole was hardly unique to the US in its contemporary setting, but it does form an important component part of the doctrine of American exceptionalism. The allure of so-called "virgin soil,"[note 1] which acted as a spur for a diverse collection of eccentrics, religious fundamentalists, speculators and aristocratic chancers in the 17th and 18th centuries, was the necessary precursor to manifest destiny.
The 19th century historian Frederick Jackson Turner established the Frontier Thesis in The Significance of the Frontier in American History, which essentially argued that the experience of the frontier created a American mythology - thus in turn creating the conception of American nationhood, which had been closely tied to continental models during its War of Independence. The only historian come President, Theodore Roosevelt, wrote that the frontier created an "American Race." Manifest destiny not only greatly increased the landmass of the American republic, it effectively created the American ideal. Without it, the United States would have been vastly different - whether this was for better or for worse is a matter of opinion.
Seeing such historical processes in isolation may lead to a critical failure of perspective. The American expansion across central continental North America is comparable in scale to the British/Canadian expansion across northern continental North America, the Brazilian conquest of Amazonia, the British/Australian colonization of the interior of Australia, and Russian expansion across northern Eurasia. Expansion is how large territorial states are created. Unable to accomplish a comparable expansion on a cute little collection of home islands with a total land mass no larger than the state of Oregon, Britain added vast territories in Asia and Africa to its empire. France did the same thing, although most of its vast territories were far less densely populated.
After the US expanded all the way to California and all along the West Coast, the frontier was officially closed and there was some debate over what to do next. The solution was to become an imperialist power, which the US did in the 1890s, attempting to seize land in the Pacific and dominate Latin American affairs.
It's possible that the idea behind manifest destiny was used to justify the expansion of Germany which kicked off the Second World War. However, it is likely that this misinterpreted (to put it nicely) German geographer Friedrich Ratzel, who studied the effect of people of Germanic origin on the development of the US in the 19th Century.