Manifest Destiny was the 19th century belief that American settlers were destined to expand across North America from the Atlantic to the Pacific ocean. This expansion often occurred by taking lands belonging to Native Americans and Mexicans, which some white Americans viewed as inferior peoples. President James K. Polk was a key proponent of Manifest Destiny, adding more territory to the U.S. than any other president through acquisitions like the Oregon Territory, Mexican Cession, and $15 million purchase of California from Mexico after the Mexican-American War.