James Knox Polk (November 2, 1795 – June 15, 1849) was the 11th president of the United States, serving from 1845 to 1849. He previously was the 13th speaker of the House of Representatives (1835–1839) and ninth governor of Tennessee (1839–1841). A protégé of Andrew Jackson, he was a member of the Democratic Party and an advocate of Jacksonian democracy. Polk is chiefly known for extending the territory of the United States through the Mexican–American War; during his presidency, the United States expanded significantly with the annexation of the Republic of Texas, the Oregon Territory, and the Mexican Cession following American victory in the Mexican–American War.

After building a successful law practice in Tennessee, Polk was elected to its state legislature in 1823 and then to the United States House of Representatives in 1825, becoming a strong supporter of Andrew Jackson. After serving as chairman of the Ways and Means Committee, he became Speaker of the House in 1835, the only person to have served both as Speaker and U.S. president. Polk left Congress to run for governor of Tennessee, winning in 1839 but losing in 1841 and 1843. He was a dark horse candidate in the 1844 presidential election as the Democratic Party nominee; he entered his party's convention as a potential nominee for vice president but emerged as a compromise to head the ticket when no presidential candidate could secure the necessary two-thirds majority. In the general election, Polk defeated Henry Clay of the rival Whig Party.

After a negotiation fraught with the risk of war, he reached a settlement with Great Britain over the disputed Oregon Country, the territory, for the most part, being divided along the 49th parallel. Polk achieved victory in the Mexican–American War, which resulted in Mexico's cession of the entire American Southwest. He secured a substantial reduction of tariff rates with the Walker tariff of 1846. The same year, he achieved his other major goal, re-establishment of the Independent Treasury system. True to his campaign pledge to serve only one term, Polk left office in 1849 and returned to Tennessee, where he died three months after leaving the White House.

Though he is relatively obscure today, scholars have ranked Polk favorably for his ability to promote and achieve the major items on his presidential agenda in his single term. He has also been criticized for leading the country into a war with Mexico that exacerbated sectional divides. A property owner who used slave labor, he kept a plantation in Mississippi and increased his slave ownership during his presidency. Polk's policy of territorial expansion saw the nation reach the Pacific coast and reach almost all its contiguous borders. He made the United States a nation poised to become a world power, but with divisions between free and slave states gravely exacerbated, setting the stage for the Civil War.