How did the belief in manifest destiny influence U.S. politics and in the 1840s?
I am not sure one can examine the period of American Westward Expansion without mentioning Manifest Destiny. Certainly, it laid the groundwork to believe in the authenticity and absolutism in American exceptionalism. When John L. O'Sullivan coined the term, it profoundly underscored and impacted the politics of the time period:
... [It is the] manifest destiny of the United States to overspread the continent allotted by Providence for the free development of our yearly multiplying millions.
The basic premise of Manifest Destiny was to assert that it was divine ordinance that the United States expand into any previously held territory and overtake it, appropriating the nation in accordance to its own subjectivity. This influenced political discourse of the time in making American interests supersede all. Polk's entrance into the Mexican War was driven by this certainty. At the same time, voices that rose to the surface in favor of political annexation were animated by the spirit of usurpation and control, believing that doing so fulfilled divine ordinance. This held incredible sway in the political discourse of the time period.