This map shows how, especially during the nineteenth century, the United States swiftly gained territory from the Atlantic to Pacific Ocean.

      What war did President James K. Polk (1795–1849) wage that led to greater territory for the United States?

      Fulfilling the Democratic phrase “manifest destiny,” Polk engaged the United States in the Mexican–American War that led to the eventual annexation of California. Tensions were already hot between the two countries after the annexation of Texas, which had begun in earnest at the end of the Tyler Administration, but completed officially in Polk’s term.

      But Polk wanted more for the country—the provinces of New Mexico and California. Mexican troops had crossed the Rio Grande and killed American soldiers in the dispute.

      Polk sent Major General Zachary Taylor (1784–1850)—his presidential successor—into the region. Taylor achieved several victories over the Mexican army at Palo Alto and Resaca de la Palma. Taylor later defeated Mexican forces at Buena Vista in 1847. American armies led by either Taylor or General Winfield Scott actually took Mexico City during the conflict.

      After the United States conquered Mexico City, Mexico realized that it should sell its land in New Mexico and California and end the war. Some speculated that the United States should simply take control of all of Mexico. This was known as the “All–Mexico” campaign. Polk faced resistance to the war from many in the country and decided to obtain peace and enlarge the country with more than a half a million square miles.

      How did President Polk obtain Oregon?

      President James K. Polk (1795–1847) refused to back down from the British and remained steadfast in his demands that the British drop claims to the Oregon territory. Polk allegedly told at least one member of Congress, “The only way to treat John Bull [another name for Great Britain] is to look him in the eye.” He acquired present-day Oregon, Washington, and part of Idaho. He obtained land up to the forty-ninth parallel, although not all the way to the 54° 40’ parallel that was desired by some in the “All–Oregon” campaign.

       What was meant by Manifest Destiny?

      The phrase Manifest Destiny was first used by American newspaper editor John O’Sullivan (1813–1895) in an article he wrote in 1845, in which he favored the annexations of Texas and Oregon. He believed that divine providence was in action and that the United States would eventually span the continent (he was right). The roots of O’Sullivan’s optimistic nationalism dated back to the Puritan era and the idea that Christian settlers had a divine obligation to spread their beliefs across the continent. Religious fervor was compounded by nationalism and feelings of racial superiority over the American Indians and, later, the Mexicans in the Southwest. But it was also an economic urge to increase the territory of the United States to take advantage of natural resources, from the fur industry to timber to farm and ranch land and, in California, gold. Eventually, it spread beyond the continental United States, leading to the entry of Alaska and Hawaii as states, and even, one could argue, the space race and the moon landing.

THE MEXICAN–AMERICAN WAR

      What caused the Mexican–American War?

      The Mexican–American War (1846–1848) was fought over the United States’s annexation of Texas. The events that led up to the conflict began in 1837 when President Jackson recognized Texas as independent (just after Texas won its war with Mexico). Republic of Texas President Sam Houston (1793–1863) felt that protection against a Mexican invasion might be necessary, so he eyed annexation to the United States. In the meantime, Mexican President Antonio Lopez de Santa Anna (1794–1876) warned that such an action on the part of the United States would be “equivalent to a declaration of war against the Mexican Republic.” In June 1844, the U.S. Senate rejected a proposed annexation treaty. But later that year Democratic Party nominee Polk, an ardent expansionist, was elected president. Because the annexation of Texas had figured prominently in his campaign platform, outgoing President Tyler (1790–1862) viewed Polk’s victory as a public mandate for annexation, and he recommended that Congress pass a joint resolution to invite Texas into the Union. Congress did so in February, and President Tyler signed the resolution on March 1, 1845, three days before leaving office.

      Mexico responded by breaking off diplomatic relations with the United States. A border dispute made the situation increasingly tenuous: Texas claimed that its southern border was the Rio Grande River, while Mexico insisted it was the Nueces River, situated farther north. In June 1846, Polk ordered Brigadier General Zachary Taylor (1784–1850) to move his forces into the disputed area. In November, the U.S. government received word that Mexico was prepared to talk. Polk dispatched Congressman John Slidell (1793–1871) to Mexico to discuss three other outstanding issues: the purchase of California (for $25 million), the purchase of New Mexico (for $5 million), and the payment of damages to American nationals for losses incurred in Mexican revolutions. This last point was critical to the negotiations, as Polk was prepared to have the United States assume payment of damages to its own citizens in exchange for Mexico’s recognition of the Rio Grande as the southern border of Texas.

      But upon arrival in Mexico City, Slidell was refused the meeting—President José Joaquín Herrera (1792–1854) had bowed to pressure, opposing discussions with the United States. When Polk received news of the scuttled talks, he authorized General Taylor to advance through the disputed territory to the Rio Grande. Meanwhile, Mexico overthrew President Herrera, putting into office the fervent nationalist General Mariano Paredes y Arrillaga (1797–1849), who reaffirmed Mexico’s claim to Texas and pledged to defend Mexican territory.

      While Polk worked through Slidell to get an audience with the Mexican government, the attempts failed. On May 9, 1846, the Cabinet met and approved the president’s recommendation to ask Congress to declare war. The next day, news arrived in Washington that on April 25, a sizeable Mexican force had crossed the Rio Grande and surrounded a smaller American reconnaissance party. Eleven Americans were killed; the rest were wounded or captured. On May 11, Polk delivered a message to Congress, concluding, “Mexico has … shed American blood upon the American soil. … War exists … by the act of Mexico herself.” By the time the war was officially declared on May 13, just more than one year after Polk had been sworn into office, General Taylor had already fought and won key battles against the Mexicans and had occupied the northern Mexico city of Matamoros.