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By this point, Mexico and the United States no longer maintained diplomatic relations, because the U.S. and Texas had signed an agreement for the annexation of Texas.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.