In theory, one aspect of this desire was its principle to bring the ideals of democratic self-government to any peoples capable of it; in practice, however, this often meant excluding Native Americans and those with non-European ancestry. Native Americans, whose usage patterns of the land were at odds with the desires of the expanding nation, represented an obstacle to the goals of Manifest Destiny. In large part, the expanding settlements and the resulting impact on the natural ecology (such as the mass slaughter of bison) were enough to push aside the Native Americans. In other cases, the indigenous inhabitants were removed through negotiation and military force by the Federal government.
Another desire was the acquisition of new lands, since land could represent potential income, wealth, self-sufficiency, and freedom. This freedom, however, often did not apply to slaveowners, who wished to take their slaves with them as part of the Westward expansion. This was an important issue in the case of Southerners who had settled in Mexico, where slavery was forbidden. Thus the cause of Manifest Destiny was used to support the seizure of land from Mexico, particularly in the case of Texas, and the resulting expansion of slavery into the acquired territory.
In the 1840s there was a strong sense that the freedoms and ideals of the United States had far reaching importance and needed to be brought to new lands, thereby broadening the nation's reach and extending its borders. It was a time of American Romanticism, an off-shoot of a more general cultural outlook that emphasized feeling, sentiment, and emotion over science and reason, serving as a reaction to the Age of Enlightenment thought of the previous generation. The world was not a static mechanism with fixed rules and boundaries, according to this new mindset, but rather an organic entity full of boundless potential, and progress could be attained through sincere belief, hard work, and bravery in the face of great risk and change. The 1830s and '40s had seen a wealth of change due to the rapid incorporation of several extremely profound technological innovations into society, including the railroad, the rotary press, and the telegraph. Religious reformation movements had spread throughout the nation (perhaps due to apprehensions and anxieties about the changes taking place), and missionary attitudes and zeal stimulated many to expand the reach of Protestant Christianity into the frontier.
Among all, belief was strong that anything could happen, and anything could be done, and much of this potential was attributed (rightly or wrongly) to the superiority of the American Way of Life. Democratic republicanism was felt to be the best form of government, and was clearly God's plan for mankind, so it was an obligation that it and freedom should be brought to as broad an area as possible. To many it seemed a clear and unavoidable destiny that would eventually reach everywhere, making the United States a leader in agriculture, industry, commerce, the arts and sciences, and all intellectual areas; "Manifest Destiny" could be thought of as an ideal of the "boundlessness of no limits" in all areas, providing a more idealistic rationale for expansion than mere ambition for land.
Several pressures motivated the realization of Manifest Destiny. Birth rates in the East had been high, since large families were considered advantageous for working farms. Immigration was increasing, due to decreased cost and risk for the ocean voyage and the perception that life would be better in the New World than in the Old. The lands of the east were strained by the increased population from these two effects, providing a strong pressure for people to move to new regions. Economic depressions in 1818, 1837, 1839, and 1841 as well as the massive failure of attempts to establish farms in the far northern colonies (such as Vermont) provided strong incentive to take the risk of moving to much more reliable farming lands in the frontier areas.
While these pressures were developing, several macro-opportunities had occurred making realization of the Manifest Destiny possible. The Native American tribes that had held the lands of the west had been decimated by disease over the past century, and so now even especially rich lands such as the Willamette Valley in Oregon were virtually void of inhabitants, providing a historically unusual opportunity to settlers of essentially free land. The discovery of South Pass in Wyoming in 1811 had established a much more feasible route across the Rockies than had been known to Lewis and Clark.
Another key opportunity arose from the massive collapse of the international fur trade. Fur trapping had been the key to opening the west to explorers and the 'mountain men' that traversed the lands to collect the animal pelts for European markets and build huge fortunes for a few early tycoons such as the Astor family. When the industry collapsed, however, the mountain men were forced to find new livelihoods; their knowledge of the western trails and experiences surviving in the wilds provided the invaluable know-how settlers required in order to succeed in the ordeal.
As the citizens of the U.S. spread westward, intense conflict with both the Native Americans and Mexico were inevitable. Already heavily depopulated due to the diseases, the Native American peoples were unable to compete against the encroaching settlers and the advanced military that accompanied them; the rapidity and force of disappearance from the West is one of the major blemishes on American history; particularly brutal episodes such as the Trail of Tears are merely examples of the widespread and systematic extermination of these people. Conflict with Mexico was more formal but also resulted in the (perhaps opportunistic) large scale acquisition of land for U.S. settlers. These two effects of Manifest Destiny have strongly colored its representation in historical hindsight; in spite of (or perhaps because of) strong belief in God and democracy, the imposition of majority rule on minorities can be horrific.
While many African Americans participated in the westward expansion, in large part the movement was strongly white-oriented, for a variety of reasons. By definition, the settlers were individuals who had enough wealth to travel but not enough to attain their desires in the East; this therefore excluded the extreme lower classes (such as ex-slaves) and the middle or upper classes (such as slave owners). The swiftness and volume of the migration also did not permit the establishment of institutions that would have fostered investors wishing to create western plantations. The availability of cheap labor in the form of immigrants also prevented the economic viability of slavery in the newly opened lands. In the northern territories there was a strong underlying distaste for slavery, ironically coupled with widespread racism, which made those areas virtually devoid of non-whites.
The subsequent effects of the Manifest Destiny through the end of the 19th century were profound, and perhaps even more far-reaching than its promoters could have anticipated. Oregon territory proved as fertile as expected (although rainier and remoter than imagined). Discovery of gold in 1849 in California and other mineral wealth elsewhere accelerated growth and the growth of several huge new industrial empires. The turmoil of the American Civil War and freeing of the slaves stimulated further migration westward to new lands, and it can be argued that incompatibilities between differing ideals of whether slavery had a part in Manifest Destiny laid at the heart of that conflict.
As outlined above, the success of the theory Manifest Destiny was due to a variety of opportunities, factors, and influences that had little to do with the democratic nature of the United States. However, this success tended to provide an ideological justification for democracy that likely helped stimulate political reformation movements the world over. On the flip-side, its ideology of the rights of a "superior" race of people to push "inferior" races aside (to the point of genocide) may also have had an insidious influence as well.
The place of the United States of America in today's world is still strongly defined by the doctrine of Manifest Destiny, even though its borders have not changed in nearly half a century. At the time of the closure of the frontier at the end of the 19th century, the U.S. turned its expansionist impulses towards a more global scale; this led to the Spanish-American War, which garnered overseas territories for the United States. The U.S. also pursued an actively interventionist policy throughout Latin America, often for the sake of promoting governments friendly to the United States. This has often led to resentment by many people in that part of the world.
Publicly, the U.S. government frequently expresses a desire and motivation to see the ideals of democracy spread the world over, and feels that successful democratization of a non-democratic nation, or at least helping preserve or add security to existing democratic nations, adds significant perceived justification to their external military activities. Privately, however, the pursuit of "American interests" have had extreme detrimental effects on peace, justice, and freedom in many, many areas of the world. This dichotomy is clearly recognized as of the same fabric as Manifest Destiny - promotion of democracy, freedom, and economic betterment, but only for some, often resulting in the abridgement of the same by others.