Mexican–American War

The Mexican–American War lasted from April 25, 1846, to February 2, 1848. It began when the United States invaded Mexico after the U.S. annexed Texas in 1845. Mexico still considered Texas its own land because it did not recognize the Treaties of Velasco signed by President Antonio López de Santa Anna. The Republic of Texas had been independent, but many people there wanted to join the United States.

The war started after U.S. President James K. Polk sent soldiers into a disputed area between Texas and Mexico. Mexican forces saw this as an attack and fought back, which led the U.S. Congress to declare war. U.S. forces moved quickly, taking control of areas such as Santa Fe de Nuevo México and Alta California. Eventually, U.S. troops marched into Mexico City in 1847.

The war ended with the Treaty of Guadalupe Hidalgo in 1848. Mexico gave up large parts of its land, including present-day Texas, California, Nevada, Utah, and parts of Arizona, Colorado, New Mexico, and Wyoming. In return, the United States paid Mexico $15 million and took on some of Mexico’s debt to American citizens. This war caused many debates in the United States over issues like slavery and led to increased tensions that later contributed to the American Civil War.

Background

Mexico after independence from Spain

Mexico gained independence from Spain in 1821 after a long struggle. The war left Mexico's main source of money—silver mining—in ruins. Mexico started as a new country but faced many problems and was not ready for big conflicts. Even though Mexico had trouble keeping control over some areas, it still tried to defend its land.

U.S. expansionism

Main article: Manifest destiny

In the early 1800s, the United States grew and wanted more land. After buying a big area called the Louisiana Purchase in 1803, the U.S. wanted to settle questions about its borders. The U.S. also wanted to buy land from Mexico to help its economy grow, especially because of the demand for cotton. Even though there were disagreements inside the U.S., many Americans believed it was their right to expand westward.

Instability in northern Mexico

Mexico's northern areas were hard to control after independence. Attacks from Native American tribes like the Comanche and Apache made life difficult for people living there. The Mexican government was busy with its own problems and could not protect these areas well. This weakness made it easier for the U.S. to later move into the region.

Foreign designs on California

Some leaders from other countries, like the United States and Britain, saw California's rich land as a good place to settle. They thought Mexico might sell or share control of California. However, Mexico still considered it their own, and there were tensions over who should control the area.

Texas revolution, republic, and U.S. annexation

Main articles: Texas Revolution, Republic of Texas, and Texas annexation

In the early 1800s, many American families moved to Texas, which was then part of Mexico. Over time, these settlers wanted more control and eventually declared independence from Mexico in 1836. After battles, Texas became its own country. In 1845, Texas agreed to join the United States, becoming the 28th state. This move caused anger in Mexico and led to conflicts between the two nations.

Prelude

Nueces Strip

After Texas won its independence from Mexico, the southern border of Texas was set at the Rio Grande River. Texans believed this meant the border was at today’s Rio Grande. However, Mexico disagreed. They said the border should be at the Nueces River instead, arguing that Texas was not truly independent and that the Rio Grande mentioned in the treaty was actually the Nueces River.

Polk's actions

In 1845, President Polk sent General Zachary Taylor to Texas with troops, positioning them near the Nueces River, ready to claim the disputed land. Polk also reached out to leaders in California, offering support for independence from Mexico but warning against European control. To avoid a conflict with Britain over Oregon, Polk agreed to divide the Oregon Country between the two nations, which upset some people in the North who wanted more land for the U.S.

Mexico's response

Mexico was unstable at this time, with frequent changes in leadership. Despite this, Mexicans felt strongly that selling land to the United States would dishonor their country. Those who suggested peace talks were seen as traitors. When President Herrera considered talking to the U.S. representative, he was removed from power. The new government firmly stated that Texas still belonged to Mexico.

Preparation for war

Challenges in Mexico

The Mexican Army faced many problems. Only a few states sent soldiers, money, and weapons to help fight. Leaders like Santa Anna tried to train volunteers, but they struggled to create a strong sense of unity or purpose. Some Mexican officers criticized Santa Anna, saying that troops were not well-trained and that important plans were unclear.

Mexican forces were split into two groups: permanent soldiers and local militia. In the north, special groups protected scattered towns. Indigenous people also played a big role in defending their land. They faced loss of resources and had to raid settlements to survive. Some joined the Mexican Army but were often treated unfairly. Raiding also let them get horses and capture people to help in battles against Mexican or American forces.

Mexican soldiers used older British muskets from the Napoleonic Wars. American soldiers started with similar weapons but later used better rifles and revolvers, giving them an advantage. The U.S. also had stronger artillery, which often decided battles.

There were also big political problems in Mexico. Different groups, like conservatives and liberals, fought for control. Presidents changed often, and leaders sometimes left to fight the war themselves. This chaos made it hard to organize a strong defense.

Challenges in the United States

The U.S. Army was smaller and needed help from volunteer regiments from different states. Some volunteers were not well-trained and behaved badly, sometimes harming civilian populations. The U.S. Navy also sent sailors to fight in California.

The U.S. Army started with eight infantry regiments, four artillery regiments, and three mounted regiments. Congress added ten more regiments for one year of service in 1847. Many soldiers were recent immigrants from many countries. Some volunteers enlisted for short periods and did not always want to stay in the war. They sometimes faced disease, danger, and difficult fighting conditions.

Political divisions in the U.S. also grew because of the war, especially over the issue of slavery. President Polk’s Democrats lost control of the House of Representatives to the Whig Party, which opposed the war. However, unlike Mexico, the U.S. managed its political fights within its government system.

Role of women

Main article: Women in the Mexican–American War

Mexican women played important roles during the war. Some women, called soldaderas, traveled with soldiers to help care for them. While they did not fight in regular battles, some joined in when their cities were defended. Others helped wounded soldiers from both sides. Two women, Doña Jesús Dosamantes and María Josefa Zozaya, are remembered for their bravery.

At home, Mexican women showed their support in different ways, such as wearing black clothing and creating serious paintings.

American women also helped during the war. Some, like Sarah Bowman, helped by giving food and caring for injured soldiers. Others stayed in Mexico to look after their businesses while their husbands were away. In the United States, women showed their support by making items for soldiers and speaking up about the war through writing and public talks. Journalists like Anne Royall, Jane Swisshelm, and Jane Cazneau helped share women's views about the war.

Outbreak of hostilities

Texas campaign

Main article: Texas Campaign

Thornton Affair

President Polk sent General Taylor and his soldiers south to the Rio Grande. Taylor built a small fort called Fort Brown/Fort Texas) near the city of Matamoros, Tamaulipas.

Mexican forces got ready for war. On April 25, 1846, a group of Mexican soldiers attacked a small group of American soldiers led by Captain Seth Thornton in an area between the Rio Grande and the Nueces River. In this event, called the Thornton Affair, the Mexican soldiers defeated the American group, and some American soldiers were hurt or captured.

Siege of Fort Texas

A few days after the Thornton Affair, Mexican soldiers began surrounding Fort Texas on May 3, 1846. They fired at the fort, and the American soldiers fired back. The fighting went on for many hours, and some American soldiers were hurt.

Battle of Palo Alto

On May 8, 1846, General Zachary Taylor arrived with more soldiers to help the fort. But a Mexican general named Arista met them near Brownsville, Texas. The American soldiers used fast-moving cannons on horses, which were very effective. The Mexican soldiers tried to fight back but were forced to move to a better position during the night.

Battle of Resaca de la Palma

The next day, on May 9, 1846, the two sides fought fiercely in the Battle of Resaca de la Palma. The American soldiers captured the Mexican cannons, and the Mexican soldiers had to retreat. Many Mexican soldiers were hurt, and some drowned trying to cross the Rio Grande. General Taylor then moved his soldiers into Mexican territory.

Declarations of war, May 1846

President Polk told Congress that Mexico had attacked American soldiers on American land, so he asked for a declaration of war. The U.S. Congress agreed to declare war on May 13, 1846. Some politicians disagreed, but most supported the war.

In Mexico, their Congress officially declared war later, on July 7, 1846.

General Santa Anna's return

After losing battles, Mexico asked General Santa Anna to come back to help. He had been living in Cuba but agreed to return to defend Mexico. However, Santa Anna secretly talked with American leaders about possibly selling land to the United States, but later denied it. He focused on leading Mexico's army instead of governing the country. Some Mexican states did not support him, and he tried to get resources for his army in any way he could.

Reaction in the United States

Opposition to the war

In the United States, opinions about the war were divided. Many Whigs, especially in the North, opposed the war. They worried that it would spread slavery into new areas. Former President John Adams spoke against the war, fearing it would bring more slave states into the country.

Exiled abolitionist Frederick Douglass also spoke out against the war, disappointed by the lack of strong opposition. Some Whigs, like Joshua Giddings and Abraham Lincoln, questioned President Polk’s reasons for starting the war. Lincoln asked for proof that the fighting happened on American soil.

Support for the war

Many Democrats, especially in the South, supported the war. They believed it was America’s destiny to expand westward. President Polk argued that Mexico had attacked American soil and that the United States needed to protect its claims in New Mexico and California.

U.S. journalism during the war

This was the first time the United States had such wide news coverage of a war. Reporters sent updates from the battlefield, and new inventions like the telegraph helped share news quickly. Writers and artists also shared their views, with some strongly supporting the war. The news helped unite Americans and celebrate victories. Generals like Taylor and Scott became popular heroes.

U.S. invasions on Mexico's periphery

See also: Mexican–American War campaigns

New Mexico campaign

After war was declared on May 13, 1846, United States Army General Stephen W. Kearny moved southwest from Fort Leavenworth, Kansas, in June 1846 with about 1,700 soldiers. His mission was to secure the lands of Nuevo México and Alta California.

In Santa Fe, Governor Manuel Armijo wanted to avoid fighting, but some local leaders forced him to prepare for battle. Armijo planned to defend at Apache Canyon but changed his mind before the American army arrived. Some leaders wanted to fight, but Armijo ordered them to stop. The local army moved back to Santa Fe, and Armijo fled.

Kearny and his soldiers arrived on August 15 and entered Santa Fe without any fighting. He claimed the New Mexico Territory for the United States and set up a new government.

After leaving for California, some people in Santa Fe planned to fight back. On January 19, 1847, they started a revolt in Taos, New Mexico, called the Taos Revolt. They attacked a leader’s home and later a mill. The U.S. army quickly moved to stop the revolt and defeated the rebels.

California campaign

Main article: Conquest of California

News of the war reached California in August 1846. Some American settlers took control of a Mexican outpost in Sonoma on June 14, 1846, and raised a flag. More joined them, and they prepared for possible fighting.

U.S. forces arrived and took control of several cities. After some battles, the fighting ended with an agreement in January 1847.

Pacific Coast campaign

Main article: Pacific Coast Campaign

U.S. ships took control of ports along the coast and cleared the area of enemy ships. They captured several places and defeated local forces.

Northeastern Mexico

Main articles: Battle of Monterrey and Battle of Buena Vista

Battle of Monterrey

U.S. troops under Zachary Taylor crossed the Rio Grande and moved toward Monterrey, Nuevo León. After a tough battle, they took the city but allowed Mexican forces to leave. Taylor later occupied another city, Saltillo.

Battle of Buena Vista

Later, Mexican forces under Santa Anna attacked Taylor’s smaller army. After a hard fight, Taylor’s troops held their position, and Santa Anna decided to withdraw.

Northwestern Mexico

In late 1846, U.S. forces moved into areas now part of northwest Mexico. They had some battles but faced little resistance from local people, who had already been affected by raids from Native groups.

Southern Mexico

The southern part of Mexico had many Native people and was far from the capital. The U.S. Navy helped by blocking ports and supporting troops. There were some battles along the coast, and local groups sometimes fought against both Mexican and U.S. forces.

Scott's invasion of Mexico's heartland

Landings and siege of Veracruz

Main article: Siege of Veracruz

President Polk sent General Winfield Scott with a second army to invade Mexico from the coast. Scott’s troops traveled by sea to the port of Veracruz to start an invasion aimed at taking Mexico City. On March 9, 1847, Scott made one of the first big sea landings in U.S. history to prepare for a siege. About 12,000 soldiers landed near the walled city using special boats. Future leaders like Robert E. Lee, George Meade, Ulysses S. Grant, James Longstreet, and Thomas “Stonewall” Jackson were part of this force.

Veracruz was defended by Mexican General Juan Morales with 3,400 soldiers. U.S. ships used bombs and guns to break down the city walls. After a 12-day siege, the city surrendered. U.S. troops lost 80 soldiers, while Mexico lost around 180, and many civilians were also hurt. During the siege, some U.S. soldiers got very sick from a disease called yellow fever.

Advance on Puebla

Main article: Battle of Cerro Gordo

Mexican leader Santa Anna let Scott’s army move inland, hoping diseases would slow them down. He picked Cerro Gordo to fight, thinking the land would help his troops. Scott’s soldiers took a different path through tough terrain and surprised the Mexicans. In the battle on April 18, the Mexican army was defeated. The U.S. lost 400 soldiers, while Mexico lost over 1,000 and had 3,000 captured. After this loss, the Mexican army kept trying to fight.

Pause at Puebla

On May 1, 1847, Scott moved to Puebla, Mexico’s second-largest city. The city gave up without fighting because people were scared of possible attacks. Scott tried to keep his soldiers from hurting people and treated locals fairly to keep peace.

Advance on Mexico City and its capture

Main article: Battle for Mexico City

Scott left a small group behind and moved toward Mexico City on August 7. After several battles, including the Battle of Chapultepec, the capital fell. During the Battle of Chapultepec, some young Mexican soldiers stayed to fight even though it was very dangerous. Their bravery became famous in Mexico.

Santa Anna's last campaign

In late September 1847, Santa Anna tried one last time to stop the U.S. Army by surrounding them. But his plan failed, and his troops scattered when they couldn’t get enough food.

Occupation of Mexico City

After taking Mexico City, U.S. forces stayed there but faced attacks from local fighters. These fighters, called guerrillas, tried to push back against the U.S. Army. General Scott protected his troops and set up posts to keep supply lines safe.

Desertions

Many soldiers in both armies left their units. In the Mexican Army, most were farmers who missed their homes and families. In the U.S. Army, about 8.3% of soldiers left. Some joined the Mexican side, especially recent immigrants from Europe. Others just wanted to go home or find better conditions.

San Patricios

The most famous group to leave the U.S. Army was the Saint Patrick’s Battalion, made up mostly of Irish and German Catholic immigrants. They left because of unfair treatment and tough conditions. Most were killed in battle, and those captured were tried and punished. Their leader, John Riley, was marked as a criminal. A memorial in San Angel remembers them today.

End of war, terms of peace

Mexico faced many challenges in defending itself during the war, as large cities and its capital were occupied. Bringing the war to an official end was difficult due to internal divisions within Mexico. Peace began in January 1847 in Alta California with the Treaty of Cahuenga, where local Mexican residents surrendered to American forces. A more complete peace treaty was needed to fully end the conflict.

The Treaty of Guadalupe Hidalgo, signed on February 2, 1848, officially ended the war. This treaty gave the United States control of Texas and established the U.S.–Mexican border along the Rio Grande. It also added large areas to the United States, including present-day California, Nevada, and Utah, as well as parts of New Mexico, Arizona, Colorado, Texas, Oklahoma, Kansas, and Wyoming. In return, Mexico received $15 million and the United States agreed to take on $3.25 million in debts owed to American citizens by Mexico. The treaty was approved by both the U.S. Senate and the Mexican government.

The lands gained by the United States are often called the Mexican Cession. The treaty also included promises to help protect people in the region from attacks, though these promises were not fully kept. The treaty marked the end of a long and difficult war between the two nations.

Main article: All of Mexico Movement

Aftermath

Altered territories

Before the Mexican-American War, Mexico claimed a huge amount of land, but by 1849, it had lost more than half of it. This included parts of what are now the U.S. states of California, New Mexico, and others. The land was often empty but had many Native American groups living there. After the war, many of these Native groups stayed in the new U.S. lands.

The new American settlers changed the laws in these areas. They kept some good ideas from Mexican law, like fair rules about sharing property and managing water. But many Mexicans and Native Americans lost important rights, like the right to vote or hold office. Some Native groups lost their land and were forced to move.

Effect on the United States

The war made many Americans feel proud of their country. It helped the idea that the United States was meant to grow and expand, called Manifest Destiny. Even though some people had opposed the war, one of the generals who fought in it, Zachary Taylor, became a president because of his success.

The war also helped train many leaders who later fought in the Civil War. Men like Ulysses S. Grant and Robert E. Lee learned important skills that they used later. Grant later wrote that he thought the war was unfair to Mexico and that it caused problems for the United States, including the Civil War.

Effect on the American military in the Civil War

Many important leaders in the Civil War had fought in the Mexican-American War. They learned how to lead soldiers and plan battles. For example, Ulysses S. Grant, who later became president, served under General Zachary Taylor and learned many useful skills. Robert E. Lee, who led the Confederate forces, also gained experience fighting in Mexico.

Social and political context

The war caused big debates in the United States about slavery. Some people wanted to prevent slavery from spreading to the new lands the United States gained. This disagreement about slavery became a major issue that later helped cause the Civil War. The war also made some Americans feel better about their country, but it also increased unfair treatment of Mexicans and Native Americans.

Veterans of the war

After the Civil War, veterans of the Mexican-American War started asking for help from the government. They wanted pensions, which are payments to help them when they got older. It took many years, but in 1887, the government finally agreed to give pensions to these veterans who had served their country.

Effects on Mexico

For Mexico, the loss in the war was very painful. The country lost a lot of land and faced many internal problems. After the war, Mexico went through more conflicts, including a war between different political groups and an invasion by France. Leaders in Mexico tried to understand why they lost and what they could do to improve. Some leaders, like Santa Anna, were blamed for the loss and later went into exile.

Legacy

Mexico

After the French left Mexico in 1867 and the liberal republic was restored, Mexico started to remember the Mexican–American War. The story of the Niños Héroes, young cadets who fought bravely at the Battle of Chapultepec, became important. Their sacrifice was honored with a small memorial in 1881. Over time, bigger monuments were built, including one at the entrance to Chapultepec Park in Mexico City, finished in 1952. By this time, the United States and Mexico were friends and worked together, even during World War II.

United States

In the United States, the war was often forgotten after the Civil War. However, some monuments were built to remember it, like one for the Palmetto Regiment in South Carolina in 1858. A painting in the U.S. Capitol Building shows U.S. soldiers entering Mexico City. The Marine Corps Hymn mentions "From the Halls of Montezuma," remembering the war. There is a cemetery in Mexico City for U.S. soldiers who died during the war, maintained by the American Battle Monuments Commission. The Mormon Battalion, a special group of soldiers, also put up monuments in Utah and Los Angeles to honor their role in the war.