The Mexican-American War was the first American military conflict fought entirely on foreign soil and the first to be closely chronicled by the press. The war ended with American victory and a treaty that increased the nation's size by more than half a million square miles. The subsequent diplomatic settlement of a controversy over the boundaries of the Oregon Territory added another quarter million square miles, creating a transcontinental nation-state. This rapid expansion of the nation's land area, coupled with dramatic military successes and developments in transportation (the railroad) and communication (the telegraph and the postal system), fueled theories of Anglo-Saxon supremacy that fused with national pride to produce Manifest Destiny, the conviction that white Americans were divinely ordained to dominate the continent, from sea to shining sea.
America's rapid acquisition of the old Mexican Southwest and the Oregon Territory seemed to mark the spectacular fulfillment of President James K. Polk's expansionistic campaign promises. Yet the issues that emerged with the acquisition of western lands, particularly in the Southwest, reawakened the sleeping giant of American politics: the controversy between North and South over slavery that most U.S. political leaders had spent a lifetime attempting to suppress. From the end of Polk's administration until the Civil War, a series of controversial compromises would follow, as the nation struggled to organize its newly acquired lands without inflaming the partisans of either slavery or abolition.
Though President Polk and Manifest Destiny embodied the spirit, adventure, and triumphalism of the period, they also paved the way for the splintering of the Union in 1861.
A narrowly elected president. A controversial war. Sharp divisions over America's foreign and domestic policy and its future path. Sound familiar? (If not, you might want to pause here and pick up a newspaper or two.) Regardless of how you feel about the current state of affairs in American politics, warfare, and foreign relations, elements of our national history can only enhance your perspective and broaden your understanding of the country's position and reputation in the world. How did we come to be the way we are? When and how did we assume the national borders that are now taken for granted on the colorful maps adorning almost every American classroom?
Study of the Mexican-American War and the expansionist policies of the mid-nineteenth-century United States can help answer many of these questions. Iraq and Vietnam were not the first controversial wars in national history. The notion that everyone in the country once zealously supported the chief executive, regardless of his party or his agenda, is a myth.
Not every square inch of American territory was gained by bloodshed, and many powerful historical figures thought that the land we did acquire was actually insufficient. If they had their way, our country would look very different today; the United States would include parts (or all) of Mexico, Nicaragua, the rest of Central America, Cuba, and a few other places to boot. Read on to learn what drove those ardent expansionists and why they ultimately failed, even though most Americans did favor aggressive territorial acquisition.
You must also remember, dear Shmoopers, that in history, nothing can be taken for granted, and hindsight is always 20/20. You should never assume that our country always looked the way it does today on a map, or—as in the ideology of Manifest Destiny—that we were destined to assume our present form. History does not neatly unfold according to a series of predetermined (or deterministic) prophecies; no matter how Americans have rationalized their land conquests, or whether they were fighting Mexicans, Native Americans, or others, these were messy, bloody, protracted battles on all sides. The outcome was neither obvious nor easy. To trivialize the process of westward expansion or imperialism as an inevitability is to oversimplify history. Even if we now know the end of this particular story—that we did in fact conquer the continent, "from sea to shining sea"—it is the how and the why that really counts. The but is also pretty important; in other words, what was the cost of westward expansion? You could think about that question in terms of the lives lost (white settlers on the frontier, American soldiers, Mexican soldiers, Mexican citizens, Native Americans); or the tribal cultures that were decimated by disease and bloodshed along the way; or the price of the many land treaties in cold hard cash; or the dramatic alterations to the landscape and the resultant changes in the environment; or the impact on national goals, values, and ideas. The Civil War must be part of that answer, too; even if most nineteenth-century Americans agreed on the virtues of expanding the national boundaries westward, they disagreed violently over whether slavery should expand into the west as well.