The expansion of the United States from 1803 to 1853 into contiguous areas such as Louisiana, Florida, Texas, the Oregon territories, and the Mexican cession is not best described as imperialism, although it contained related elements. This expansion involved lightly populated areas in which the influx of settlement from the older portions of the nation soon constituted the great bulk of the inhabitants. The Northwest Ordinance of 1787, a profoundly anti-imperialist measure, had early defined the process by which such areas could be divided into prospective states and ultimately brought into the union as equal members. The resulting expansion represented the continuous extension of a single society over vast neighboring areas, rather than the takeover of one society by another.
The North American continent was not unoccupied; indigenous Indian tribes were found in every part of it, while the areas taken from Mexico contained many scattered settlements, particularly in California and around Santa Fe. Neither the people nor the government of the United States showed much interest in such preexisting societies; the aim of the United States was to brush them aside and replace them with the society and culture of the incoming majority. This was particularly true in regard to the Indians; rather than take over Indian society, the whites virtually destroyed it. The process was tragic for its victims, and Americans' constant assertions that they were peopling an empty continent contained the seeds of hypocrisy. There were nevertheless important differences between the movement of such a settlement frontier and the establishment of a true empire. For example, while the United States acquired half of Mexico's national territory between 1845 and 1848, the transfer entailed less than 2 percent of the Mexican population. Broadly speaking, the Mexican War was fought to gain territory, not a captive people, and the land thus gained would be populated largely from the existing United States. For purposes of comparison, the activities of the British in India, where they ruled a teeming alien society, and the British in Australia, where they settled a continent and built a self-governing nation, were so dissimilar that the use of a single term to describe both cases does more to obscure than enlighten. Prior to the Civil War, American expansion came closer to the Australian example, though dispossessing a more numerous indigenous people, and the end result cannot be accurately classified as imperialism.
There were, of course, common features in the earlier expansion and later imperialism of the United States. Chief among these were a strong sense of national mission and special destiny, a general confidence in the unique superiority of American institutions, a belief in the inequality of races and peoples, and the very habit of expansion itself. The expansionism of "manifest destiny" could lead toward true imperialism, as in the abortive movement to annex all of Mexico during the Mexican War. If westward expansion was not the same as imperialism, it furnished some of the materials out of which the latter could grow.