James K. Polk developed his political career as a Tennessee lawyer and supporter of Andrew Jackson. He entered the state legislature in 1823 and the United States House in 1825, becoming a prominent advocate of Jacksonian policies. He chaired the Ways and Means Committee and served as Speaker from 1835 to 1839. Elected governor of Tennessee in 1839, he lost subsequent campaigns before emerging unexpectedly as the Democratic presidential nominee in 1844.

Polk's campaign emphasized territorial expansion, including annexation of Texas and American claims in Oregon. He defeated Henry Clay and took office in March 1845 after Texas annexation had already been authorized under John Tyler. His administration settled the Oregon boundary with Britain at the forty-ninth parallel in 1846, avoiding war in the northwest. Domestically he secured the lower Walker Tariff and restoration of an independent treasury, pursuing a relatively focused program of measures during a single term.

Relations with Mexico deteriorated over Texas and competing boundary claims. Polk sent forces under Zachary Taylor into the disputed region between the Nueces and Rio Grande, and fighting in 1846 became the basis for his request for war. Critics challenged his account that Mexico had attacked on American soil. United States forces occupied extensive Mexican territory, and the Treaty of Guadalupe Hidalgo in 1848 transferred California and much of the Southwest while recognizing the Rio Grande boundary. These gains involved military conquest and dispossession, with profound consequences for Mexican residents and Indigenous peoples.

The new territories also intensified the dispute over the extension of slavery. Polk, himself a slaveholder, did not resolve the conflict, and proposals such as the Wilmot Proviso exposed divisions that outlasted his presidency. He worked closely with his cabinet and kept detailed records while overseeing a demanding wartime administration. Having pledged not to seek a second term, he left office in 1849 and died only a few months later. His career was unusually effective at achieving stated policy goals, but the scale of territorial acquisition cannot be assessed apart from the war's disputed justification and its lasting sectional consequences. His presidency helped create the continental dimensions of the United States while deepening conflicts over sovereignty, race, and the political future of slavery.