Parts of the US That Used to Be Part of Mexico

narcolepticnerd September 20, 2025 0 comments

Overview

Large areas of the present-day United States were once part of Mexico before the Mexican-American War (1846–1848). These lands were ceded to the US by Mexico in the Treaty of Guadalupe Hidalgo (1848) and the Gadsden Purchase (1853). The transfer of these territories had a profound impact on the development of the United States, the fate of indigenous peoples, and the relationship between the US and Mexico.

Historical Background

Spanish and Mexican Rule

Before Mexico gained independence from Spain in 1821, much of the American Southwest was part of the Spanish Empire. After independence, Mexico inherited these vast northern territories, which included present-day California, Nevada, Utah, Arizona, New Mexico, Texas, and parts of Colorado, Wyoming, Oklahoma, and Kansas. These regions were sparsely populated by Mexican settlers, indigenous peoples, and a small number of American immigrants.

Texas Revolution and Annexation

One of the earliest flashpoints was Texas. In the 1820s and 1830s, American settlers moved into the Mexican province of Texas, eventually outnumbering Mexican residents. Tensions over cultural differences, slavery (which Mexico had abolished), and governance led to the Texas Revolution (1835–1836). Texas declared independence and became the Republic of Texas. In 1845, the US annexed Texas, which Mexico still considered its territory, setting the stage for war.

The Mexican-American War (1846–1848)

The annexation of Texas and disputes over its southern border (the US claimed the Rio Grande, Mexico claimed the Nueces River) led to the outbreak of the Mexican-American War in 1846. The US, under President James K. Polk, pursued a policy of “Manifest Destiny”—the belief that the US was destined to expand across the continent. American forces invaded and occupied large parts of northern Mexico, including Mexico City.

Treaty of Guadalupe Hidalgo (1848)

The war ended with the Treaty of Guadalupe Hidalgo, signed on February 2, 1848. Under the treaty:

Mexico ceded about half its territory to the US, including present-day California, Nevada, Utah, most of Arizona, half of New Mexico, and parts of Colorado and Wyoming.
The US paid Mexico $15 million and assumed $3.25 million in debts owed by Mexico to American citizens.
The treaty guaranteed property rights and civil rights to Mexican citizens in the ceded territories, though these promises were often not honored in practice.

The Gadsden Purchase (1853)

In 1853, the US purchased an additional strip of land in southern Arizona and New Mexico from Mexico for $10 million. This area, known as the Gadsden Purchase, was needed for a southern transcontinental railroad route.

States (All or Part)

California
Nevada
Utah
Arizona
New Mexico
Texas
Colorado (southern part)
Wyoming (southwestern part)
Oklahoma (western tip)
Kansas (southwestern part)

Impact on Indigenous Peoples

The transfer of these territories did not take into account the many Native American nations who lived there. The US government soon imposed its own policies, leading to the displacement, marginalization, and in many cases, violent removal of indigenous peoples. The expansion westward brought settlers, railroads, and new laws that further eroded indigenous land and sovereignty.

The Slavery Debate and Sectional Conflict

The acquisition of new territories reignited fierce debates over the expansion of slavery. The question of whether slavery would be allowed in the new lands polarized the North and South, contributing to the breakdown of national political parties and the eventual outbreak of the Civil War. The Compromise of 1850, which admitted California as a free state and allowed other territories to decide the issue by popular sovereignty, was a direct result of the Mexican Cession.

Economic and Demographic Changes

The discovery of gold in California in 1848 triggered the Gold Rush, bringing hundreds of thousands of settlers west and accelerating California’s admission as a state in 1850. The new territories attracted immigrants from around the world, as well as Americans seeking land and opportunity. The region’s economy shifted from ranching and subsistence farming to mining, agriculture, and trade.

US-Mexico Relations

The loss of so much territory was a national trauma for Mexico, shaping its politics and national identity for generations. The border established by the Treaty of Guadalupe Hidalgo and the Gadsden Purchase remains the US-Mexico border today. Issues of migration, trade, and cultural exchange continue to be influenced by this shared history.

Legacy and Modern Significance

Today, the former Mexican territories are among the most diverse and dynamic regions of the United States. The cultural influence of Mexico is evident in place names, architecture, cuisine, language, and traditions throughout the Southwest. Many Mexican Americans trace their roots to families who lived in these lands long before they became part of the US.

Key Events Timeline

1821: Mexico gains independence from Spain.
1836: Texas declares independence from Mexico.
1845: US annexes Texas.
1846–1848: Mexican-American War.
1848: Treaty of Guadalupe Hidalgo signed; Mexican Cession.
1850: California admitted as a state.
1853: Gadsden Purchase.