Polk was a chief influence In the House of Representatives helping Andrew Jackson with his Bank policy. Supporters knew Polk was the candidate who stood for progress especially in including Texas into the union which was popular in the South and also with issue of obtaining Oregon which was favored by the North. Polk was also in favor of acquiring California. In his position on Oregon he seemed to be risking war with Great Britain. Acquisition of the California are was far more difficult as the president sent an offer to Mexico with a $20,000,000, plus settlement of damage claims owed to Americans, in return for California and the New Mexico country. Congress declared war and, despite opposition, supported the military operations.

To Mexican troops this was not well received, and they attacked Taylor's forces when they arrived. American forces won victories and occupied Mexico City. Finally, in 1848, Mexico ceded New Mexico and California in return for $15,000,000 and American assumption of the damage claims. Polk was also in favor of an independent treasury system. He was very persuasive in getting what he wanted in the areas of domestic, diplomatic, and military policy. Probably no president ever worked more diligently at his job than Polk, and when his term ended he was near exhaustion. He settled into a quiet retirement in Nashville, he fell ill only three months after leaving office and died there on June 15, 1849.