Mexican-American War: Causes, Outcomes, and Legacy

Introduction

The Mexican-American War was fought between 1846 and 1848 and marked the first war in which the United States fought primarily on foreign soil. The war was initiated by the United States, with President Polk seeking to expand American territory under the doctrine of Manifest Destiny. This doctrine held that the United States should spread across all of North America, and it was used as justification for military actions such as this one. The major outcome of the war was a massive expansion of the United States across much of what is now the American Southwest. The U.S. absorbed New Mexico, Arizona, Nevada, Utah, and California as the result of this conflict — an acquisition that continues to shape America in many ways today (History.com, 2016).

Background and Origins of the War

At the outbreak of the war, Mexico held much of the territory that now comprises the U.S. Southwest. Mexico was, however, a weak country. Its government was headquartered in Mexico City, removed from these territories by thousands of miles of desert. Mexico had been founded in 1821 after a war of independence from Spain. The capital, Mexico City, sat in a heavily populated and agriculturally rich region that had been home to the Aztec Empire. Mexico had sought to encourage migration to Texas but failed in that regard, and Texas consequently saw more English-speaking settlers. Texas would soon break away from Mexico, as would the neighboring Republic of the Rio Grande. For several years, ongoing tension existed between the United States and Mexico over the status of Texas. Mexico retained its claim to the territory, while Texans preferred annexation by the United States.

James Polk ran for president on a platform built around Manifest Destiny and specifically intended to annex Texas if he won the election, which he did. This was followed in 1845 by the official annexation of Texas by the United States. Polk sought to take over much of what is today the Southwest and the Oregon Territory, and moved to do just that. Initially, he offered to purchase much of this land, but the offer was rejected. His response was to move troops into a disputed area that was part of the state of Coahuila (History.com, 2016).

By this point, Mexico and the United States no longer maintained diplomatic relations, because the U.S. and Texas had signed an agreement for the annexation of Texas. Mexican forces were also active in the disputed area, and the U.S. declared war on Mexico on May 13, 1846, marking the official beginning of the conflict (Department of State, 2016). Mexico's military was relatively weak, and few of its forces were stationed along the border area. The United States was therefore able to secure a series of victories owing to its drastically superior manpower in the early stage of the conflict.

There were three fronts at this point in the war. Stephen Kearny captured Santa Fe, Zachary Taylor invaded Mexico from the north, and Commodore Sloat invaded California from the sea. All three commanders won their respective engagements. Even as the Americans pushed southward, the Mexicans did not surrender. General Winfield Scott led a mission to capture Mexico City, landing at Veracruz and marching inland from there (Smithsonian, 2016). Mexico City was captured in September 1847.

The Treaty of Guadalupe-Hidalgo

The war ended in February 1848 with the signing of the Treaty of Guadalupe-Hidalgo. This treaty ceded vast amounts of Mexican territory to the United States, including most of what is now the Southwest. The United States paid Mexico $15 million for this territory. The treaty proved controversial, however, because by that time expansionist sentiment had grown considerably. Expansionists wanted the United States to annex all of Mexico. This represented a new dimension of Manifest Destiny, and many within Congress — including members of Polk's own Democratic Party — agitated for an expansion of the conflict to seize the entire country.

Thus, the decision to sign the Treaty was contentious. On one hand, it vastly extended American territory and avoided dragging the nation into a prolonged war in more populous and geographically difficult southern regions. Polk saw little advantage in continuing the conflict, even though he himself was an expansionist (U.S. State Department, 2016). The Treaty was signed, the war ended, and the map of the United States was dramatically reshaped.

Conclusion

The Mexican-American War came about as a result of Manifest Destiny. Essentially, the United States was the dominant power closest to the northern Mexican territories, and it believed those territories should be American. A border dispute centering on the self-declared republic of Texas served as a pretext for a multi-pronged invasion of Mexico. The military contest concluded quickly, with a decisive victory for the United States. Despite domestic pressure to seize the entire country, President Polk recognized that such action would require a far longer and bloodier conflict, and instead signed the Treaty of Guadalupe-Hidalgo, ending the war. The Treaty gave the U.S. its pathway to the Pacific, fulfilling a critical element of Manifest Destiny, while effectively ending any southern territorial ambitions.

The fallout from the war unfolded across several fronts. Polk quickly negotiated the Treaty of Oregon to establish the framework for the modern western borders of the United States. This provided the security needed to begin promoting settlement of the West in earnest, including policies affecting Native Americans and the drive to fill the vast land between the Mississippi River and the Pacific Ocean. The eventual discovery of oil in both Texas and California would carry these regions well beyond their agricultural roots — though those agricultural foundations would themselves prove a valuable and enduring resource base. The United States is also a far more Spanish-influenced country than it would have been without the annexation of the southwestern states. These regions form a unique cultural zone that makes a lasting contribution to American identity — one the country would otherwise never have had. In this sense, the Mexican-American War was both the beginning of the end of Manifest Destiny and the foundational event that shaped the modern United States, setting the stage for the massive westward settlement that followed.