James K. Polk was the 11th President of the United States, serving from 1845 to 1849. He was a member of the Democratic Party and is known for his strong belief in Manifest Destiny, the idea that the U.S. was destined to expand across the continent. Polk's presidency is marked by significant territorial expansion, including the acquisition of Oregon Territory and the Mexican-American War. During the Mexican-American War, which lasted from 1846 to 1848, Polk aimed to acquire California and New Mexico. His administration successfully negotiated the Treaty of Guadalupe Hidalgo, which resulted in the U.S. gaining vast territories, including present-day California, Arizona, New Mexico, and parts of Colorado, Utah, and Nevada. Polk chose not to seek re-election, fulfilling his promise to serve only one term.