James K. Polk (1795-1849) was the eleventh president of the United States. His name is perhaps most closely associated with Manifest Destiny, as the term was coined by a fellow Democrat in 1843, the year before he began his presidency. Manifest Destiny—the belief that Americans were destined by God to conquer the continent to the Pacific Ocean—soon came to embody the governing philosophy of the Polk administration and its ardently expansionist aims.
Polk successfully campaigned on an expansionist platform and vowed not to compromise with the British on the dispute over the Oregon Territory's northern border. Two days after he took office, however, diplomatic relations between Mexico and the United States were severed over the American annexation of Texas. Tensions continued to escalate as Polk settled the Oregon boundary with Britain at 49ºN, far south of the initial demand of "54º40' or fight," angering northern Democrats who were jealous of the imminent territorial gains to the south. In May 1846, Polk convened his Cabinet and obtained its approval for sending a message of war against Mexico to Congress. The entire premise for the war was controversial from the start and widely decried by Polk's Whig opponents in Congress, though most of them did not dare vote against bills to provide supplies for the troops. In March 1847, Polk sent Nicholas P. Trist, the chief clerk in the State Department, to Mexico along with Gen. Winfield Scott's troops in order to commence treaty negotiations. Against Polk's orders and amidst great unrest, Trist signed the Treaty of Guadalupe Hidalgo in early February 1848. Polk had wanted more land cessions—in addition to California and the Southwest—but was forced to "settle" for what Trist's treaty got him: California, Nevada, Utah, most of New Mexico and Arizona, and the disputed regions of Texas. In all, it was the largest single land acquisition since the Louisiana Purchase in 1803.