Final answer:
President James K. Polk pursued U.S. territorial expansion in the mid-19th century, advocating for Manifest Destiny and leading the country through the Mexican-American War to gain considerable land in the West.
Step-by-step explanation:
President James K. Polk was the leader who sought to expand U.S. territory during the mid-19th century. Elected in 1844, Polk was a firm believer in the concept of Manifest Destiny, which was the idea that the United States was preordained to occupy the North American continent from the Atlantic to the Pacific. Under his administration, the United States saw significant territorial growth. This expansion was achieved through various means including the annexation of Texas, the procurement of Oregon territory, and most notably, the Mexican-American War, which resulted in the United States acquiring large swaths of land that include present-day California, Nevada, Utah, Arizona, and parts of Colorado, Wyoming, New Mexico, and Texas.