By 1820, the United States already extended well beyond its original boundaries. Through the Louisiana Purchase of 1803 and treaties with Spain and Britain, the nation's borders moved west to the Rocky Mountains, north to the 49th parallel, and south to Florida and the Gulf of Mexico. These boundaries remained essentially intact until the 1840s, when the United States acquired massive territories in the Southwest and on the Pacific Coast.
A complex mix of political, social, and economic factors fueled American expansionist sentiment in the 1840s. Many Americans subscribed to the concept of "Manifest Destiny," the belief that Providence preordained the United States to occupy as much land on the continent as possible. Some saw lucrative economic opportunities in the vast stretches of arable land and superb Pacific Coast ports. Others dreamed of the romance of settling uncharted terrain, or thought the United States should expand rapidly across the continent before foreign nations could do so. These expansionist yearnings fueled American settlement in Texas and Oregon, the acquisition of which became a principal object of American foreign policy by 1845.
The strained relationship between the Texans and the Mexican government turned violent in 1835 with the Texan Revolt. After a series of bloody engagements, including the legendary siege of the Alamo, the Texans, led by Sam Houston, won a decisive victory at San Jacinto in 1836. Following that battle, the Mexican army commander General Antonio López de Santa Anna signed, but later renounced, treaties that granted Texas independence and established its southwest boundary at the Río Grande.
While a majority of Texans and many Americans favored annexation in 1836, the admission of a slave-holding Texas (or several states formed from Texas territory) threatened the delicate balance of slave and free state representation in the Senate that had been carefully maintained since 1820. Rather than upset this balance, the United States recognized Texas as a sovereign nation, leaving the tensions between Mexico and Texas to simmer for the nine and one-half years of Texan independence.
The Oregon settlers from the United States and Britain were very different groups. The British were chiefly fur traders associated with the Hudson's Bay Company, while the Americans were a more eclectic lot. American settlement began in the 1830s when Protestant missionaries moved into the Wilamette Valley. Their accounts of the fertile soil of the region spread rapidly to the East and spurred a massive migration of thousands of American families westward along the Oregon Trail. The resulting population disparity along with an overall decline in the fur trade, convinced the British government to work for a negotiated settlement to the Oregon issue.
As with Texas, popular opinion over the Oregon Country was divided. Whereas Texas territory would have added proslavery representation in Congress, any potential states formed from the Oregon Country would be free states. Accordingly, Northerners were the chief advocates of acquiring as much Oregon Country as possible.
James K. Polk and the Policy of Expansion
Negotiated Settlement in Oregon
Negotiations between the United States and Britain over the Oregon Country began in the summer of 1845. The initial American proposal called for the boundary to be drawn at the 49th parallel, bisecting Vancouver Island. When British negotiators rejected this proposal, President Polk took a bolder position by reasserting his campaign promise to support the 54° 40' line and announcing the American intent to terminate the joint occupancy agreement within a year. While expansionist Northerners cheered these provocative actions with shouts of "Fifty-Four Forty or Fight!," Southerners in Congress made it clear that they would not risk war with Britain over Oregon.
British leaders were similarly adverse to conflict and did not want to jeopardize their important economic relationship with the United States. In June 1846, the Senate, preoccupied with war against Mexico, quickly approved the Oregon Treaty with Britain, setting the boundary at the 49th parallel.
During the nearly two years of war, American troops took possession of Mexican territory in what is now California, New Mexico, northern Mexico, and ultimately the capital, Mexico City. The Treaty of Guadalupe Hidalgo, which ended the war in February 1848, established dramatically altered boundaries for the United States and Mexico. Under the terms of Guadalupe Hidalgo, Mexico ceded to the United States an immense territory of nearly one million square miles, including land in what is now California, Nevada, Utah, New Mexico, Arizona, Colorado, Wyoming, Oklahoma, and Kansas. The United States, in turn, agreed to pay Mexico $15 million and assume $3.25 million in debt claims against Mexico. The treaty also provided that Mexican people who remained on their lands would be granted American citizenship and allowed to retain their property.
The Legacy of Expansion
By 1848, the Manifest Destiny championed by many Americans had been realized. The territory of the United States stretched from the Atlantic to the Pacific Coasts and from the 49th parallel to the Río Grande. Although they had been granted citizenship rights, many Mexican people who remained on their lands found the new American property laws complicated and confusing. Some unscrupulous American settlers used their greater familiarity with the law to acquire huge tracts of land owned by Mexicans.
The fertile territory acquired during 1840s also contained the bitter seeds of discord. Many Northerners and Southerners disagreed sharply over the slavery status of the territories, and eventually this dispute flamed into the carnage that was the Civil War. Today, the legacy of nineteenth century U.S. expansion is evident in the dynamic cultural and political interchange between the United States and Mexico. The people of each nation influence each other's language, food, music, and traditions, while their leaders continue to work together to resolve a host of political and environmental questions, including complex immigration, water rights, and land use issues.