The United States acquired an immense territorial tract during James Polk's presidency, 1845–1849. A determined expansionist, Polk attached 1.2 million square miles to the US domain, a feat effected in three phases.

First, the Texas republic, independent of Mexico since 1836, became the Union's twenty-eighth state in 1845. This event, aided by Polk's predecessor, President Tyler, caused the government in Mexico City to sever diplomatic ties with the United States. Texas in the mind of Mexican officialdom was a province in rebellion, belonging to the state of Coahuila, and unavailable to any foreign power. Yet the Mexican minister to Washington, Juan N. Almonte, could only protest the US annexation of Texas. His country's political fragility prevented decisive counter-action.

Second, the simmering dispute between London and Washington over the Oregon country, provoking Polk's bluster and threats of another British–US–Canadian clash (“54–40 or fight”), was resolved peacefully in June 1846. Settlement via the Buchanan–Parkenham treaty spared the administration from strife in the northwest while contemplating a Mexican adventure. Agreement with Britain also entailed US frontage on the Pacific and secured lands that later encompassed three states: Oregon, Washington, Idaho (bits of Montana).

Most dramatic, by right of conquest in 1846–1848, Polk presided over the addition of Mexican holdings to the United States. This cession exceeded 40 percent of the Mexican republic.