James K. Polk

Polk was a firm believer in the individual sovereignty of states and he felt that federally-supported internal improvements would make the states beholden to the federal government.

Several of America's most popular leaders at that time had been war heroes–George Washington and Andrew Jackson, in particular–and the generals knew that their quickest ticket to political success lay in securing a stunning victory in Mexico.

Manifest Destiny was a phrase used by politicians in the 1840s to explain and justify continental expansion by the United States all the way across to the Pacific Ocean. By giving the country a "mission"–to expand the liberties and freedom of America all the way across the country–they were able to garner support for Westward expansion, the Mexican War, and other expansionist ideas.