because of thomas jefferson and napolean

I wasn't joking....... I remeber High School History... You must have went to LS......

http://en.wikipedia.org/wiki/Manifest_Destiny

Manifest Destiny is a phrase that expressed the belief that the United States was destined to expand from the Atlantic seaboard to the Pacific Ocean; it has also been used to advocate for or justify other territorial acquisitions. Advocates of Manifest Destiny believed that expansion was not only good, but that it was obvious ("manifest") and certain ("destiny"). It was originally a political catch phrase or slogan used by Democrats in the 1845-1855 period, and rejected by Whigs and Republicans of that era. Manifest Destiny was an explanation or justification for that expansion and westward movement, or, in some interpretations, an ideology or doctrine which helped to promote the process. This article is a history of Manifest Destiny as an idea, and the influence of that idea upon American expansion.

The phrase "Manifest Destiny" was first used primarily by Jackson Democrats after 1845 to promote the annexation of much of what is now the Western United States (the Oregon Territory, the Texas Annexation, and the Mexican Cession). The term was partly revived in the 1890s, this time with Republican supporters, as a theoretical justification for U.S. expansion outside of North America. Historian David Potter concludes that in 1854 the Ostend Manifesto and the Kansas-Nebraska Act were "the two great calamities of the Franklin Pierce administration.... Both brought down an avalanche of public criticism." More important says Potter, they permanently discredited Manifest Destiny and popular sovereignty. [1]

The term fell out of usage by U.S. policy makers early in the 20th century, but some commentators believe that aspects of Manifest Destiny, particularly the belief in an American "mission" to promote and defend democracy throughout the world, continued to have an influence on American political ideology.[2]