Westward expansion of the United States of America had a number of very significant ramifications for both the newly-emerging nation and for the peoples and lands that were incorporated into the United States.
One major effect of westward expansion was to catapult the United States from a fledgling former British colony into a major country with which the colonial powers Britain, France and Spain would soon have to contend. President Thomas Jefferson’s successful 1803 purchase from France of a huge tract of land – the Louisiana Purchase – instantaneously increased the size of the United States by 828,000 square miles, while providing the country with a major extension of its territory into the continent’s west. Also, with the purchase of Louisiana, the United States now controlled the city of New Orleans and its strategically important port, which controlled entry into the Mississippi River from the south. With control of access to the Mississippi, American trade with foreign countries could be more efficiently conducted.
Another major effect of westward expansion was the challenge it posed to Spain and its hold on territories west of the Mississippi River. With France no longer controlling Louisiana, and with the British on the retreat south of Canada, there was concern that Spain would react to American encroachments in a hostile manner, precipitating another war. Spain, concerned about U.S. expansion, had already nullified a treaty with the United States that had guaranteed U.S. access to the entire length of the Mississippi in 1798. France’s decision to sell the entire expanse of land that included both ends of the river thoroughly undercut Spain’s leverage against the United States.
A third major effect of westward expansion was the further marginalization of the native populations and their eventual decimation. As Americans expanded westward, they increasingly came into direct conflict with Native American tribes, particularly the Plains Indians across the mid-west. As treaties were systematically made and broken with native tribes, and as the native tribes were forced off of land vital to their existence and onto reservations where they had less control over their means of survival, their numbers declined and their cultures were destroyed.
As the United States continued its westward expansion and settled territories along the coast of the Pacific Ocean, it began to emerge as a major power, although it would be many decades before that power was realized and utilized on a global scale. The natural resources subsumed under the country’s expansion were put to use in the service of industrialization and the country’s extension to the Pacific facilitated a new horizon for trade. To the extent that annexation of the Hawaiian Islands could be seen as the ultimate manifestation of westward expansion, then the effect of that expansion on the New World was to make the United States a Pacific power, with all the benefits that eventually accrued from that status. The country’s destiny would become entwined with the Pacific to the same extent that it became entwined with Europe across the Atlantic.