- Date:
- 1848
- Participants:
- Mexico
- United States
Mexican Cession
What was the Mexican Cession?
What was the impact of the Mexican Cession on the slavery debate in the U.S.?
How did the Mexican Cession affect Mexico?
How is the Zimmermann Telegram connected to the Mexican Cession?
Mexican Cession, relinquishment of more than half of Mexico’s territory to the United States under the terms of the Treaty of Guadalupe Hidalgo, which ended the Mexican-American War in 1848. The ceded territory covered more than 525,000 square miles (1,360,000 square km), including land that is now part of the U.S. states of New Mexico, Utah, Nevada, Arizona, California, Texas, Wyoming, Oklahoma, Kansas, and Colorado. Though annexed through war, the U.S. paid Mexico $15 million for the land and took over its debts to U.S. citizens, which amounted to about $3 million. The debate over whether the new states in this region would permit slavery fueled sectionalism and was one of the issues that led to the American Civil War.
Background: The Mexican-American War
Texas had secured independence from Mexico in 1836, and it was made a U.S. state in 1845, which resulted in Mexico severing diplomatic ties with the United States. The border between Texas and Mexico had been debated since Texas’s independence, and the dispute now continued between the United States and Mexico: The Americans considered the Rio Grande the legitimate border, while Mexico insisted that the Nueces River, lying 150 miles (about 240 km) to the northeast, was the boundary.
U.S. Pres. James K. Polk, whose demands to annex Texas and expand the country westward had won him the 1844 election, moved troops into this disputed territory and simultaneously attempted to start diplomatic negotiations to acquire the disputed borderland as well as Mexican territory in what is now California and New Mexico. He used news of a skirmish between Mexican and American troops in April 1846 to gain congressional support for a declaration of war against Mexico. The mostly one-sided war ended with the U.S. capture of Mexico City in September 1847. While many Americans, including Polk, were in favor of a more extensive acquisition of Mexican territory, discontent in the U.S. Army was considerable, with desertions on the rise as soldiers succumbed to disease (more died from infection and poor hygiene than from combat). That meant Polk had to be pragmatic and end the war before the rising human cost of the conflict spiraled out of control. The subsequent Treaty of Guadalupe Hidalgo, signed on February 2, 1848, and approved by the U.S. Senate a month later, placed the U.S.-Mexico border at the Rio Grande and led to Mexico giving up land to the United States.
Legal and human impact of the land cession
The United States paid $15 million (worth about $660 million in 2026) to Mexico for the acquired land; in addition, it took responsibility for more than $3 million in claims made by U.S. citizens against Mexico. The treaty specified that a survey team, comprising both Americans and Mexicans, would demarcate the border between the two countries. The Gila River and the Rio Grande were both part of this new border.
The annexed territory covered more than 525,000 square miles (about 1,360,000 square km, or 336,000,000 acres). This included the whole of California, Nevada, and Utah, southwest Wyoming, western Colorado, Arizona north of the Gila River, and the western portion of what is now New Mexico, bordered by the Rio Grande to the east.
Mexicans residing in the areas now annexed by the United States were given a choice to stay or move to Mexico, with the former option granting them the opportunity to become U.S. citizens. The U.S. government pledged to protect their rights and lands, including from attacks by Native peoples, and promised them freedom of religion. This provision of the treaty had its detractors among Americans, though—specifically, those who were unhappy with the United States absorbing a considerable number of Roman Catholics, many of whom were not of exclusively European descent. Law enforcement also proved to be poor and unreliable, and these new citizens were often victims of violence and discrimination.
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The U.S. Senate specifically struck out Article X in the treaty, which would have protected land rights granted by the Mexican government in the territory now belonging to the United States. Doing so gave the U.S. government the power to validate the legitimacy of existing land grants, which also made it easier for new settlers to acquire land. A later law continued to change land ownership in the region: Passed in 1851, it required Mexican landholders in California to confirm their land deeds with the U.S. government, but lengthy legal proceedings coupled with illegal land seizures and westward migrations driven by the California Gold Rush resulted in landholders, especially those unfamiliar with American laws and taxes, either losing their land or falling into debt.
Aftermath and legacy
U.S. Maj. Gen. Zachary Taylor, who played a key role in the Mexican-American War and became a national hero, succeeded Polk as president in 1849. The unresolved question of how slavery would be managed in these new U.S. states was addressed through the Compromise of 1850 (adopted in late 1850 after Taylor’s death in July), which temporarily settled the issue but contributed to tensions between free states in the North and slaveholding states in the South that would eventually culminate in the American Civil War. However, U.S. expansion continued through the 19th century in line with the notion of Manifest Destiny.
Calls for a southern transcontinental railroad through the newly acquired territory produced, in 1853, the Gadsden Purchase (known in Mexico as the sale of the Mesilla Valley), by which the United States acquired from Mexico 29,670 square miles (76,845 square km) south of the Gila River, now in southern Arizona and southern New Mexico, for $10 million (about $450 million in 2026).
The events of the Mexican Cession left a long-lasting legacy in both countries. In Mexico the loss of territory discredited the country’s conservative government and left many of its citizens unsure of their country’s future as an independent state. After the fall of dictator Antonio López de Santa Anna in 1855, the new liberal government’s proposed land and social reforms led to La Reforma, a civil war between the government and the conservative clergy and military. While the liberals eventually won in 1860, conflicts between these factions would continue in Mexico until 1876.
The Zimmermann Telegram, a coded message sent in 1917 by German foreign secretary Arthur Zimmermann to the German minister in Mexico, proposed an alliance between Mexico and Germany in the event of the United States joining World War I against Germany and expressed German support for Mexico in recovering the land lost in the Mexican Cession. The intercepted telegram contributed to the entry of the United States into World War I.
In the 1960s Texas-born activist Reies Tijerina led a movement of descendants of the original Mexican landholders, demanding restoration of their rights to land that was part of the Mexican Cession. Peaceful protests continued for months, until members of the movement, including Tijerina, were arrested for land occupation and an assault on government officials.