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. 