Do you know which war was the first American war fought on (mostly) foreign soil? The Mexican-American War was fought on land formerly under Mexico's jurisdiction. Keep reading about how the annexation of Texas, the Rio Grande river, and Manifest Destiny combined to lead to this brief yet the impactful war.
Fig. 1: A Painting of the Battle of Molino del Re
The Mexican-American War of 1846
The Mexican-American War began in the spring of 1846 as tensions heightened between the United States and Mexico regarding the border between the two countries. Keep reading to learn about the causes and consequences of this war.
Timeline of the Mexican-American War
Below is a brief timeline that outlines some major events in the Mexican-American war.
| Approximate Date | Event |
| 1821 | Mexico became independent of Spain and founded the province of Texas |
| 1835 | Americans living in Texas founded a provisional government, prompting the Texas Revolution. |
| 1836 | Americans in Texas declared independence from Mexico |
| 1845 | Texas became the 28th state when the United States annexed it. |
| 1846 | The start of the Mexican-American War. |
| 1848 | The end of the Mexican-American War. The Treaty of Guadalupe Hidalgo was signed. |
Causes of the Mexican War
One of the causes of the Mexican-American was the Republic of Texas' new-found freedom. Texas was under the control of the Spanish before becoming Mexican territory in 1821. It was sparsely populated until the Mexican government encouraged occupation by white settlers. Revolution began in Texas, and the Texas Revolution ran from 1835 to 1836. Not only was Texas fighting for its independence, but it was simultaneously requesting annexation by the United States. In December 1845, President Polk signed legislation that annexed Texas, making it the 28th state.
Annex:
Take control of a territory or area near yours, often by using force
This led to the primary cause of the war, which was a border dispute between Mexico and the United States. President Polk believed in Manifest Destiny, the idea that God's plan was for Americans to take and settle new territory. Polk wanted to expand American territory into modern-day California, New Mexico, and other parts of the southwest. Polk first offered to pay for the land. When Mexico rejected his offer, he sent troops into the border dispute area.
Manifest Destiny:
The idea that God's plan was for Americans to take and settle new territory
In April 1846, Mexican troops attacked and killed several American soldiers stationed in the border dispute area. From there, the Mexican troops turned their focus to Fort Texas, participating in an unsuccessful siege against the Americans. Two more subsequent battles-Battle of Palo Alto and Resaca de la Palma-found American soldiers triumphant against the Mexican armed forces.
In May 1846, President Polk and the United States Congress declared war against Mexico.
Fig. 2: American President James Polk
The Mexican-American War
The area of dispute, the land north of the Rio Grande, was sparsely populated. This led to quick and easy success for the American troops looking to conquer the land.
Mexico agreed with an exiled general, General Santa Anna. Santa Anna agreed to make a false alliance with President Polk. This alliance resulted in Santa Anna's double-crossing Polk taking control of the Mexican army. Santa Anna quickly led the Mexican army into a conflict, engaging in the Battle of Buena Vista in February 1847. The Mexican army fell to American forces.
This was not the only battle happening at this time. American troops successfully invaded and occupied Veracruz. Mexican forces could not stand up to American forces; however, they did launch successful attacks on American supply lines. Santa Anna, who had become the Mexican president, resigned.
By September 1847, the Mexican army was defeated. Mexico's capital, Mexico City, had fallen. After Mexico surrendered, the American and Mexican governments began negotiating the terms of a treaty.
The Treaty of Guadalupe Hidalgo was signed in February 1848.
Fig. 3: General Santa Anna
Consequences of the Mexican-American War
The Treaty of Guadalupe Hidalgo formally established the Rio Grande as the Mexican-American border. Mexico was forced to recognize the annexation of Texas officially. Mexico had to sell much of its territory to the United States.
The United States received over 500,000 square miles of land from Mexico. The absorption of this land and its people would forever alter the lives and culture of the United States.
While the battle with Mexico was over, the war's end signaled another battle in the United States. Americans began to argue over slavery in the new land. This was one of the causes of the American Civil War.
Manifest Destiny is the idea that God's plan was for people to conquest and settle on new territory. In the United States, Manifest Destiny was one of the most significant motivators behind American expansionism.
In the 1840s, the United States grew, with an increased need for more land for farming, business, and growing families. Americans look to the land west of the Louisiana Purchase for expansion.
People viewed its expansion to the West as America's manifest destiny, believing God wanted them to settle the land and spread democracy and capitalism to the Pacific Ocean. This justified Americans for conquering others' land and warring with other nations. White Americans saw themselves superior to the indigenous people living on desired soil. Native people were simply an obstacle to be removed through any means necessary.
James Polk, president from 1845-1849, was a tremendous champion of manifest destiny. Under Polk, the American federal government annexed Texas and participated in the Mexican-American war. Polk was also behind the 1846 Oregon Treaty. Polk obtained a large chunk of land for the federal government (modern-day Idaho, Oregon, and Washington) and avoided going to war simultaneously.
Fig. 4: A Map of the Operations of the American Army in the Valley of Mexico in August and September 1847
Treaty of Guadalupe Hidalgo
The Mexican-American War, which the United States fought from 1846-1848, ended with the Treaty of Guadalupe Hidalgo. The treaty was named after the town where the treaty was signed.
The Treaty of Guadalupe Hidalgo outlined some consequences for Mexico.
- Mexico had to surrender its claim to Texas
- Mexico had to relinquish over half of its territory to the United States, also known as the Mexican Cession.
- This includes land in California, Nevada, New Mexico, Utah, Colorado, Arizona, New Mexico, and more.
- Mexico had to acknowledge the Rio Grande as the new border
The American government also had some obligations.
The U.S. government had to pay Mexico money
They had to pay off Mexican debt owed to American citizens
The US government agreed to extend rights to Mexican citizens living within its new borders
Did you know? The International Boundary Commission was created to mark the boundary between the two countries.
Fig. 5 The Last Page of the Treaty of Guadalupe Hidalgo
Aftermath of the Mexican War
The Treaty of Guadalupe Hidalgo, which officially ended the war, was signed in February 1848. It brought forth the official end of the Mexican-American war. As a result, the United States absorbed a large percentage of Mexico's land.
The treaty created legal protection for Mexican citizens currently living in the newly ceded land that chose to remain where they were over moving within the revised Mexican borders.
After the war came to an end, Mexican citizens experienced instability and violence. This was the catalyst for Mexican immigration into the United States.
Fig. 6 Mexican-American War Uniform
Mexican American War Summary
The Mexican-American War began in the spring of 1846. Some causes of the war include:
the annexation of Texas
a border dispute regarding the Rio Grande river
President Polk's desire to harness Manifest Destiny and expand into Mexico's land
Mexico's refusal to sell land to the United States
Mexico attacked American soldiers stationed near the Rio Grande
Mexico initiated a siege against American troops at Fort Texas
American troops deployed to the area around the Rio Grande. They were successful in several battles. The American army successfully occupied Veracruz and triumphed in the Battle of Buena Vista.
The Mexican Army was defeated by September 1847. After negotiations, the United States and Mexico signed the Treaty of Guadalupe Hidalgo. The treaty had explicit obligations for each participating country.
The Treaty of Guadalupe Hidalgo outlined some requirements for Mexico.
- Mexico had to surrender its claim to Texas
- Mexico had to relinquish over half of its territory to the United States, also known as the Mexican Cession.
- This includes land in California, Nevada, New Mexico, Utah, Colorado, Arizona, New Mexico, and more.
- Mexico had to acknowledge the Rio Grande as the new border
The American government also had some obligations.
The U.S. government had to pay Mexico money
They had to pay off Mexican debt owed to American citizens
The US government agreed to extend rights to Mexican citizens living within its new borders
The end of the Mexican-American War led to Mexican immigration to the United States. The instability and violence in the region pushed many Mexican immigrants north into the United States.
| Approximate Date | Event |
| 1821 | Mexico became independent of Spain and founded the province of Texas |
| 1835 | Americans living in Texas founded a provisional government, prompting the Texas Revolution. |
| 1836 | Americans in Texas declared independence from Mexico |
| 1845 | Texas became the 28th state when the United States annexed it. |
| 1846 | The start of the Mexican-American War. |
| 1848 | The end of the Mexican-American War. The Treaty of Guadalupe Hidalgo was signed. |
The Mexican War - Key takeaways
- The Mexican-American War began in the spring of 1846. Some causes include the annexation of Texas, a dispute over the American/Mexican border, and the American pursuit of Manifest Destiny.
- The Mexican Army was defeated in September 1847.
- After negotiations, the United States and Mexico signed the Treaty of Guadalupe Hidalgo in 1848.
- The Treaty of Guadalupe Hidalgo forced Mexico to cede a large amount of its land. It insured that the United States offered protection and rights to its former citizens living within American borders.
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