End of the U.S.-Mexican War

The U.S.-Mexican War ended in 1848 with a major victory for the United States. U.S. forces had won a series of battles in northern Mexico and California and eventually captured Mexico City, the capital of Mexico. With its capital occupied and its armies badly weakened, Mexico agreed to negotiate peace.

On February 2, 1848, representatives from the United States and Mexico signed the Treaty of Guadalupe Hidalgo. The treaty officially ended the war.

One major result of the treaty was the settlement of the Texas-Mexico border dispute. Mexico recognized the Rio Grande as the southern border of Texas.

This was important because the United States and Mexico had disagreed about the border before the war. The United States claimed that Texas extended south to the Rio Grande, while Mexico argued that Texas ended at the Nueces River.

The treaty settled the dispute in favor of the U.S. claim.

The treaty also transferred a vast amount of Mexican territory to the United States. This land included present-day California, Nevada, and Utah, as well as most of New Mexico and Arizona and parts of Colorado and Wyoming.

This transfer became known as the Mexican Cession. It greatly increased the size of the United States and helped fulfill the goal of many supporters of Manifest Destiny, who wanted the nation to expand across North America.

In return, the United States agreed to pay Mexico $15 million and take responsibility for certain claims that U.S. citizens had made against the Mexican government.

The territorial loss was enormous for Mexico. The war and treaty resulted in Mexico losing a large portion of the territory it had controlled before the conflict.

The treaty did more than change lines on a map. Thousands of Mexican citizens were living in the lands transferred to the United States.

The Treaty of Guadalupe Hidalgo promised protections for the property and civil rights of Mexican residents in the transferred territories. People living there could choose to move into Mexico or remain where they lived and eventually become U.S. citizens.

In practice, however, many Mexican Americans later faced discrimination and challenges to their land ownership. The transfer of territory therefore changed not only national borders but also the government and laws under which thousands of people lived.

Indigenous nations also lived throughout the lands claimed by both Mexico and the United States. The treaty transferred control between the two governments without the consent of these nations. U.S. expansion into the region would lead to increased pressure on Indigenous lands and further conflicts in the years that followed.

For the United States, the war brought enormous territorial gains. But gaining so much land also created a major political problem: Would slavery be allowed in the new territories?

The United States was already divided over slavery. Many white Southerners wanted slavery to be allowed to expand westward. Many Northerners opposed its expansion into the new territories. Each side also worried about how new states could change the balance of political power in Congress.

Even before the war ended, Representative David Wilmot proposed banning slavery from any territory gained from Mexico. His proposal, known as the Wilmot Proviso, did not become law, but the debate surrounding it showed how strongly the issue of slavery was dividing the country.

Over the next decade, Americans continued to argue over whether slavery would expand into western territories. These disagreements became one of the major causes of the Civil War.

The U.S.-Mexican War also influenced the military leaders who would later fight in the Civil War. Officers including Ulysses S. Grant, Robert E. Lee, and Thomas “Stonewall” Jackson gained military experience during the conflict. Less than fifteen years later, some of these men would command opposing armies during the Civil War.

The war changed both the United States and Mexico. The United States gained an enormous amount of western territory and established the Rio Grande as the Texas-Mexico border. Mexico lost control of a large part of its northern territory. People already living in those lands found themselves under a new government, and Indigenous nations faced increased U.S. settlement on their lands.

The war also intensified the national argument over slavery. In that way, the end of the U.S.-Mexican War settled one major conflict over territory while helping create another political crisis that would shape the future of the United States.