How do you account for the huge territorial expansion of the

Many southern Americans had migrated ito texas when it was part of Mexico so the South was keen to make it part of the Union. The North however, despite the threat of Britain were unsure as extending with a state the size of Texas would increase the "slave power" element and give a lot more power to the south. Eventually they annexed Texas in 1844. There were both civil and national reasons why Texas should be annexed, but they both encompassed national security and civil harmony.
             Upon James Polk's election in 1844 the idea of Manifest Destiny had sparked a renewed interest (after Louisiana purchase there was interest also) in the expansion of the United States. Polk used the "Manifest Destiny" ideal as part of his campaign, a campaign which Polk had based around promise to expand into Oregon. Because Polk was elected to be president, he at the least represents the majority of the American public. His election shows both the political aims for expansion but also helps to show the people's desire for expansionist policy.
             Even before Polk got into power there was still a large degree of political intrigue over expansion, in the west especially. Prior President, Tyler and his secretary of State John C Calhoun had an idea to expand. Calhoun had a mandate from the people which was very pro-expansionism. .
             The desire for good land and the migration which went hand in hand with this was a driving factor in the territorial expansion. Agriculturally plentiful areas like Oregon and California (more popular in the late 1840's after the discovery of gold in 1848) attracted large numbers of settlers , as settlers moved west it was important that there still was some sort of laws and morals. It seemed that where the people went the government was obliged to follow and thus the country expanded territorially.
             Part of the reason for Americans spreading was to flee persecution as well as for monetary desire or will to find land.