United States Territorial Expansion This information describes the history of territorial expansion of the United States. 1 An 1845 magazine article claimed that other nations have undertaken to intrude themselves into it [U.S, territorial expansion], ... for the avowed object of thwarting our policy and hampering our power, limiting our greatness and, checking the fulfillment of our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions." The concept of the United States having a manifest destiny promoted the idea that territorial expansion was necessary and inevitable. 2. Past territorial acquisitions contributed to this belief. The 1783 Treaty of Paris ended the Revolutionary War and moved the nation's western border to the Mississippi River. The Louisiana Purchase doubled the size of the country. The Treaty of 1818 established the 49th parallel as the border between the U.S. and Canada. Che year later, Spanish cessions gave the nation territory on the Gulf of Mexico and 3 One year after the magazine article, a treaty with Great Britain divided the disputed Oregon Territory and extended the U.S. Canadian border along the 49th parallel. The signing of an 1848 treaty ended a war and added territory that would eventually make up all or part of seven western states. With the purchase of a nearly 30,000 square mile portion of Mexico in 1853, the borders of the continental United States were set.