During James K. Polk's presidency, foreign policy revolved around the U.S. desire for additional territory in North America. Even before the Revolutionary War, Americans had looked westward, and in the early years of the republic the United States had expanded its borders toward and then beyond the Mississippi River. Whether through a congressional joint resolution, negotiations, purchase, or war, President Polk by the end of his term intended for the United States to stretch from coast to coast, firmly in possession of the Oregon Territory and California.

Annexation of Texas

President Tyler, in the last months of his term, boldly sent a joint resolution to Congress for the annexation of Texas. A resolution required only a simple majority vote in both houses rather than the two-thirds majority in the Senate that is normally required for a treaty. Tyler sent Congress a resolution because he knew that the two-thirds vote in the Senate was not to be had. Congress passed the joint resolution a few days before Polk's inauguration in March 1845. This allowed Texas to bypass the territorial stage and come into the union as the fifteenth slave state in December 1845. Although Mexico had promised war against the United States if it annexed Texas, no war followed. But when Texas moved its militia into disputed territory west of the Nueces River, thereby staking a claim to the Rio Grande as its southern border, Mexico responded by breaking off diplomatic relations with the United States.

Oregon

During the 1844 campaign, Polk had pledged to settle the boundary of the Oregon Territory with Britain, and once in office, he moved quickly to acquire sole title to Oregon. His supporters in the 1844 campaign had promoted the occupation of the entire territory, as encapsulated in their slogan (which Polk did not disavow), "54°40' or Fight." Both Great Britain and the United States had jointly occupied this region since 1818, and it was clear that Polk wanted the west coast of North America for the United States, possibly even including Mexican-controlled California. In the beginnings of negotiations, Polk bluffed to Britain that he wanted all the territory up to 54°40'. In the end, the President's shrewd but unseemly bluster earned him a compromise rather than a war with the British. In spite of his own supporters' more extreme demands, Polk agreed to a boundary at the 49th parallel, giving the United States present-day Oregon, Idaho, and Washington, as well as control of the Columbia River.

War with Mexico

Polk now turned the entirety of his attention toward Mexico. Much was at stake in the area. Great Britain had been discussing with Mexico for months the possibility of buying California. The British previously had offered to support the independence of Texas in return for the abolition of slavery in the area. Even before settling the Oregon question, Polk had moved troops into the disputed territory just north of the Rio Grande and sent a special envoy, John Slidell, to Mexico. Slidell carried with him a U.S. offer to buy California as well as plenipotentiary powers to settle disputed border claims. Slidell's arrival in Mexico triggered a revolt against the Mexican president—who had indicated a willingness to deal with Slidell—by army officers who pledged to recover the "stolen province of Mexico."

In late April 1846, Mexican troops crossed the Rio Grande and killed eleven U.S. soldiers. In response, Polk requested a declaration of war from Congress, arguing that Mexicans had "shed the blood of our fellow-citizens on our own soil." By May 13, 1846, both nations officially were at war. Most Whigs opposed the war, as did Calhoun, but all remembered how the nation had turned on the Federalists following their opposition to the War of 1812. Hence, only fourteen members of the House and two senators voted against the declaration. Abraham Lincoln, a first-term Whig congressman from Illinois, condemned the war as an "unconstitutional" and aggressive act, even challenging Polk to take him to Texas and show him "the spot" on which Mexicans had shed American blood. This position proved unpopular with his western constituents and figured into his decision not to run for a second term.

Within seven months, the U.S. Army completely defeated the much larger Mexican Army on its own soil in three triumphant military campaigns. The first phase was conducted by General Zachary Taylor's four-thousand-man army in northern Mexico. Engaging much larger forces, Taylor earned the nickname "Old Rough and Ready" for his victories at Palo Alto and Resaca de la Palma and, after being reinforced by several thousand volunteers, his capture of Monterrey in September 1846.

Phase Two commenced under General Stephen Watts Kearny, who meanwhile led an army of fifteen hundred regulars and fighting frontiersmen west from Fort Leavenworth to Santa Fe in New Mexico, occupying it on August 18, 1846. Half of Kearny's army then fought their way through the Mexican province of Chihuahua, marching three thousand miles to link up with Taylor's army at Monterrey in the spring of 1847. The other half of Kearny's forces joined American settlers in California under the command of Captain John C. Fremont, who had captured Sonoma and declared California an independent republic. Their flag, displaying the picture of a grizzly bear, gave rise to the term "bear-flag revolt."

The third phase of the war had all the markings of a comic opera. In July 1846, Polk provided safe passage into Mexico for a former Mexican army officer who had been overthrown in a palace coup in 1844 and exiled to Cuba—the infamous General Antonio López de Santa Anna. Commander of Mexican forces at the Battle of the Alamo, Santa Anna was a man hated by Texans and distrusted by his own countrymen. He promised Polk that he would make peace on American terms in return for a payoff of $30 million. When Santa Anna arrived in Mexico City, however, the new government named him supreme commander of the army and president of the republic. He immediately raised a new army and marched north in early 1847 to attack Taylor's force at Monterrey. In the meantime, Polk was growing increasingly worried about Taylor's popularity. Angry with the general for having declared an armistice without his approval after capturing Monterey, Polk transferred half of Taylor's army to General-in-Chief Winfield Scott, whom Polk ordered to lead an invasion of central Mexico.

Taylor, a Whig who suspected political intrigue on Polk's part, met in battle Santa Anna's fifteen-thousand-man army at Buena Vista on February 22, 1847. When the smoke cleared, Taylor's five thousand Americans had defeated Santa Anna's army in a fierce battle marked by the courageous counterattack of a Mississippi regiment commanded by young Jefferson Davis. When news of Taylor's victory reached back home, his popularity soared and Whigs began publicly to mention his name as a possible candidate for the presidency.

In a daring and unprecedented amphibious landing, Scott captured the port of Veracruz in March 1847. Then, in one of the riskiest field maneuvers in the books, he launched a five-month, hard-fought campaign over the two hundred miles to Mexico City. Most European military strategists predicted disaster, because not only was Scott's army outnumbered three to one, it was cut off from its supply bases, filled with ill-trained volunteers, and operating in unknown terrain. But in the end, and after bloody hand-to-hand fighting, Scott's army stood in possession of Mexico City on September 14, 1847.

With Mexico's capital in American hands, Polk sent diplomat Nicholas Trist to negotiate the terms of Mexican surrender with yet another new government, this one having overthrown Santa Anna after his loss of Mexico City. Expansionist fever at home in his own party pressured Polk to wring every possible concession from Mexico. Some even called for the annexation of "All Mexico," although all Polk really wanted was California. Trist resisted Polk's instructions, however, and so the President recalled him. In spite of this, the diplomat continued to negotiate with Mexico, and on February 2, 1848, he signed the Treaty of Guadalupe Hidalgo, which authorized U.S. payment of $15 million for California and New Mexico, and named the Rio Grande as the Texas border. With the treaty in hand, Polk wisely decided to submit it to the Senate. After a short debate, the Senate approved the treaty on March 10, 1848, by a vote of thirty-eight to fourteen. Half of the opposition came from Democrats who wanted more Mexican territory, and half from Whigs who wanted none at all. Mexico, in what was called the Mexican Cession, ceded over one-third of its territory to the United States, increasing the latter's size by one-fourth. This Mexican Cession now contains the present-day states of Arizona, Utah, Nevada, California, much of New Mexico, and portions of Wyoming and Colorado. Just before leaving office, Polk created the Department of the Interior in an effort to help organize and administer these vast new western lands.

American deaths in the Mexican War surpassed 13,000, although only 2,053 of this number died in battle or from wounds received in combat—the rest died from disease. Another 4,100 Americans were wounded. Mexico suffered nearly 50,000 casualties. The Mexican War was the first war covered by large numbers of the press, and Polk's reliance on volunteers gave the war a democratic character. Influential American men of letters like Walt Whitman and James Fenimore Cooper saw it as the beginning of a world movement to extend democracy.

Treaty of New Granada

Concerned that Britain might use the war with Mexico to expand its claims in Central America and the Caribbean, Polk responded positively to the initiative of New Granada (present-day Colombia) for a commercial treaty. The agreement, signed by U.S. Minister Benjamin A. Bidlack, conveyed to the U.S. the right of way across the Isthmus of Panama. In return, the United States promised to guarantee the neutrality of the isthmus and the sovereignty of New Granada. In so doing, Polk paved the way for the eventual construction in 1914 of the Panama Canal, although a canal connecting the Atlantic and Pacific oceans already found boosters within Polk's cabinet. Meanwhile, Polk opened discussions with Spain with the aim of purchasing Cuba, although in this case his goal was to prevent another war of annexation being pushed by certain fellow Democrats. However, Spain was not interested in the U.S. offer.