A term in the Tennessee legislature and seven consecutive terms in the U.S. House of Representatives provided James K. Polk with a solid political background. A Democratic Party leader, he backed Jackson's policies against the Bank of the United States. As Ways and Means Committee Chairman, he led the defense of Jackson's policy favoring some state banks. Polk won election as Tennessee governor in 1839, but at a time with the Whig Party was increase its strength in the state. He lost his bid for re-election in 1841 and 1843.

A dark horse at the 1844 Democratic convention, he emerged as the presidential candidate. Running on a platform of "the reoccupation of Oregon and the reannexation of Texas," Polk defeated Henry Clay in the election. Polk pledged to govern independently as president and his policies caused much dissension within his own party. He had attempted to negotiate with Mexico for the purchase of land.

With the annexation of Texas, Mexico would not receive the U.S. envoy. While Polk was making plans to seek a declaration of war, a Mexican force crossed the Rio Grande River and attacked U.S. forces under Zachary Taylor. Polk did not have confidence in Taylor and sent Gen. Winfield Scott to land at Veracruz and march on Mexico City. Also dispatched was Nicholas P. Trist to complete a peace treaty. Unhappy with the progress, Polk recalled Trist. Trist ignored the recall and negotiated a treaty that provided for the cessation of California and New Mexico and recognition of the U.S. claim to Texas. All of these were objectives Polk had before the war.