One of the major reasons for territorial expansion was the American's will to conquer what ever they could. This confidence came from the ideal of "Manifest destiny." Coined by a New York paper in 1845 the term described American's "God given right" to take what they wanted, and that Providence had intended American's or the United States to control all of North America. This belief not only allowed for, but also encouraged expansion and furthermore there was ample justification for moving Native Americans and Mexicans off their lands. It was this main factor which gave license to political and social aims.
National security was a smaller factor in accounting for the expansion. By the 1840's surrounding the United States were Canada, Mexico, A now independent Texas and the Oregon County. Oregon and Mexico were pretty much on offer, because of the vast semi- populated area in Oregon and because Mexico was relatively unthreatening, however they were still a force and it was important to have some security. It was obvious that the U.S. feared Britain especially in Canada but there was still a desire to take as much as possible from Britain. Britain was also involved along with France in Texas , this was a little too close to home and the annexation of Texas was probably most necessary for national security. This belief dates back as far as the Monroe Doctrine of which plainly stated that there be no European colony in the United States. Territorial expansion was a must for defence of the nation and to prevent any European involvement in North America.
The involvement of France and Britain in negotiations with Texas over annexation was a conflicting issue for the United States, however the real conflict was between North and South. There were concerns about Britain taking Texas, which would have put a stop to western expansion all together. Aside from this there was a civil issue, the South wanted Texas desperately.