Polk was the last strong pre-Civil War president, having met during his four years in office every major domestic and foreign policy goal set during his campaign and the transition to his administration.

General Zachary Taylor, who had served in the Mexican–American War, won the Whig presidential nomination and defeated Polk's preferred candidate, Democratic Senator Lewis Cass.

Van Buren and the Whig frontrunner, Henry Clay, opposed the annexation and a potential war with Mexico over the disputed territory,[4] Polk and former President Andrew Jackson strongly supported the acquisition of the independent country.

[11] After learning of his nomination, Polk promised to serve only one term, believing that this would help him win the support of Democratic leaders such as Cass, Wright, John C. Calhoun, Thomas Hart Benton, and James Buchanan, all of whom had presidential aspirations.

[16] The Whigs blanketed the nation with hundreds of thousands of anti-Polk tracts, accusing him of being a puppet of the "slaveocracy" and a radical who would destroy the United States over the annexation of Texas.

[27] In selecting a new cabinet, Polk generally heeded Jackson's advice to avoid individuals who were themselves interested in the presidency, though he chose to nominate Buchanan for the crucial and prestigious position of Secretary of State.

[34] After the outbreak of the Mexican-American War, Polk shook up his cabinet, sending Bancroft to Great Britain as an ambassador, shifting Mason to his old position of Navy Secretary, and successfully nominating Nathan Clifford as Attorney General.

Polk's attempt to fill Baldwin's seat became embroiled in Pennsylvania politics and the efforts of factional leaders to secure the lucrative post of Collector of Customs for the Port of Philadelphia.

As Polk sought to find his way through the minefield of Pennsylvania politics, a second position on the high court became vacant with the death, in September 1845, of Justice Joseph Story; his replacement was expected to come from his native New England.

[43] Due to disagreements regarding the extension of slavery, Senator Benton of Missouri and Secretary of State Calhoun disagreed on the best way to annex Texas, and Polk became involved in negotiations to break the impasse.

[45] Polk also retained the United States Ambassador to Texas, Andrew Jackson Donelson, who sought to convince the Texan leaders to accept annexation under the terms proposed by the Tyler administration.

[51] Edward Everett, President Tyler's minister to Britain, had informally proposed dividing the territory at the 49th parallel with the strategic Vancouver Island granted to the British.

[53] Though both the British and the Americans sought an acceptable compromise regarding Oregon Country, each also saw the territory as an important geopolitical asset that would play a large part in determining the dominant power in North America.

[58] In his December 1845 annual message to Congress, Polk requested approval of giving Britain a one-year notice (as required in the Treaty of 1818) of his intention to terminate the joint occupancy of Oregon.

[64] In June 1846, Pakenham presented an offer to the Polk administration, calling for a boundary line at the 49th parallel, with the exception that Britain would retain all of Vancouver Island, and British subjects would be granted limited navigation rights on the Columbia River until 1859.

[68] Polk's willingness to risk war with Britain had frightened many, but his tough negotiation tactics may have gained the United States concessions from the British (particularly regarding the Columbia River) that a more conciliatory president might not have won.

[70] In the 1830s and 1840s, the United States, like France and Britain, sought a reparations treaty with Mexico for various acts committed by Mexican citizens and authorities, including the seizure of American ships.

[81] As successful negotiations with the unstable Mexican government appeared unlikely, Secretary of War Marcy ordered General Taylor to advance to the Rio Grande River.

[82] Polk had been advised by Alejandro Atocha, an associate of Santa Anna, that only the threat of war would allow the Mexican government the leeway to sell parts of Mexico.

[84] In a message to Congress, Polk explained his decision to send Taylor to the Rio Grande, and stated that Mexico had invaded American territory by crossing the river.

[93] The next day, Taylor led the army to victory in the Battle of Resaca de la Palma, eliminating the possibility of a Mexican incursion into the United States.

[100] Army Captain John C. Frémont led settlers in Northern California in an attack on the Mexican garrison in Sonoma, beginning the Bear Flag Revolt.

[120] In April 1847, Scott defeated a Mexican force led by Santa Anna at the Battle of Cerro Gordo, clearing the way for a march on Mexico City.

[132] The two sides also agreed to the right of Mexicans in annexed territory to leave or become U.S. citizens, American responsibility to prevent cross-border Indian raids, protection of church property, and a $15 million payment to Mexico.

[148] The agreement helped to establish a stronger American influence in the region, as the Polk administration sought to ensure that Great Britain would not dominate Central America.

[181] The 1848 Whig National Convention took place on June 8, with Taylor, Scott, Henry Clay, and Massachusetts Senator Daniel Webster emerging as the major candidates.

These views were long reflected in the historical literature, until Arthur M. Schlesinger Jr. and Bernard De Voto argued that Polk was nobody's tool, but set his own goals and achieved them.

"[194] Though there were powerful forces compelling Americans to the Pacific Ocean, some historians, such as Gary Kornblith, have posited that a Clay presidency would have seen the permanent independence of Texas and California.

"[196] Whig politicians, including Abraham Lincoln and John Quincy Adams, contended that the Texas Annexation and the Mexican Cession enhanced the pro-slavery factions of the United States.

[197] Unsatisfactory conditions pertaining to the status of slavery in the territories acquired during the Polk administration led to the Compromise of 1850, one of the primary factors in the establishment of the Republican Party and later the beginning of the American Civil War.