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Causes and Key Events of Mexican-American War of 1846

The Mexican-American War, a crucial conflict in the mid-1800s, took place amid intricate geopolitical tensions and land disputes between the United States and Mexico. The war started in 1846 and ended in 1848. It not only shaped the histories of both nations, but also gave way to further American expansion and westward movement. 

The fundamental causes of the conflict were closely tied to manifest destiny, regional expansion concerns, and differing views on the annexation of Texas. Through this blog, we will shed light onto the complex factors leading to the war and understand the impacts of the war on US history. So let’s dive right in. 

Historical context of the Mexican-American War

The Mexican-American War occurred amid a complicated historical background. It was characterized by enduring conflicts and land disputes between the United States and Mexico. Strained relations between the two nations emerged in the 1800s when Mexico gained independence from Spanish rule in 1821. 

Mexico’s expansive territory included areas like present-day Texas, California, New Mexico, Arizona, and parts of other southwestern states. Fueled by the idea of Manifest Destiny and a quest for more land, the U.S. set its sights on these Mexican-controlled regions. It wanted to take over these lands and delve deeper into the southern parts. 

Not only that, Mexico and America also differed on many other issues. Political and cultural differences added to the tensions. Mexico operated under the tradition of centralism. It struggled to unify its many diverse regions into one. On the other hand, The United States was driven by republicanism and wanted to expand westward. 

The matter of Texas’s independence and its subsequent annexation by the U.S. exacerbated hostilities. Read ahead to know why the Annexation of Texas was the leading cause of the Mexican-American war. 

Causes of the Mexican-American War

On the face, it looks like there was only one cause that led to the war in 1846. In reality, there were a plethora of factors that contributed to the dispute. Here are the most important ones. 

1. Annexation of Texas and Border Disputes

The Mexican-American War was significantly triggered by the annexation of Texas, a key factor leading to the conflict. Texas, having declared independence from Mexico in 1836, sought to join the United States in 1845. However, Mexico strongly opposed this annexation, claiming rightful ownership of Texas. This annexation disagreement heightened tensions, emerging as a critical trigger for the conflict.

2. Manifest Destiny and its Influence on U.S. Expansion

The war was strongly influenced by Manifest Destiny, the belief that the United States was destined to expand westward. Fueled by this conviction, many Americans considered territorial expansion a fundamental right and duty. The annexation of Texas and the American desire for additional Mexican territories showcased how Manifest Destiny shaped U.S. policies. It also propelled an assertive push for westward expansion.

3. Economic and Political Drivers

Many economic factors were also involved. The quest for access to California’s valuable ports, played a vital role. The U.S. sought control of California to enhance trade prospects and secure Pacific Coast ports. Politically, the dispute over the extension of slavery into new territories heightened tensions. The delicate balance between slave and free states was a central political concern, and the acquisition of new territories fueled heated debates on the expansion of slavery.

4. Cultural and Ideological Variances

Cultural and ideological disparities between the United States and Mexico were pivotal in escalating the conflict. The cultural gap between Anglo-American settlers in Texas, with distinct linguistic and cultural backgrounds from the Mexican government, strained relations. Additionally, the clash between U.S. republicanism and Mexican centralism deepened ideological conflicts, making diplomatic resolutions challenging.

Also Read: Total War: The Transformative History of Warfare 

Key Events Leading to the War

1. Annexation of Texas (1845)

The addition of Texas to the United States in 1845 proved pivotal in setting the stage for the Mexican-American War. Securing independence from Mexico in 1836, Texas sought entry into the Union. President John Tyler championed annexation, and Congress endorsed the resolution.

However, tensions heightened with Mexico who claimed that Texas was part of their country. The annexation significantly strained relations between the two nations, becoming a key factor leading to the impending conflict. 

2. Thornton Affair (1846)

The Thornton Affair, unfolding in 1846, became a crucial event intensifying hostilities between the United States and Mexico. A clash near the Rio Grande involved American troops led by Captain Seth Thornton and Mexican forces. 

Resulting in American casualties, the skirmish escalated border disputes. The Thornton Affair prompted the U.S. government to escalate military involvement. This ultimately culminated in the declaration of war.

3. U.S. Occupation of Disputed Territory

Subsequent to the Thornton Affair, the United States occupied contested land between the Rio Grande and the Nueces River. This region was at the heart of conflicting claims regarding Texas’s southern border. The U.S. military presence heightened tensions, viewed by Mexico as a violation of its sovereignty and a direct provocation. This occupation marked a crucial phase leading up to formal hostilities.

4. Declaration of War (1846)

In May 1846, President James K. Polk urged Congress to declare war on Mexico, citing issues stemming from the Thornton Affair, occupation of disputed territory, and broader concerns of unpaid claims and reparations. 

On May 13, 1846, Congress endorsed a declaration of war against Mexico. This formal proclamation signaled the official start of the Mexican-American War, initiating significant military engagements and shaping the course of North American history.

Major Battles and Campaigns

1. Siege of Fort Texas (May 1846)

The Siege of Fort Texas, also known as the Siege of Fort Brown, happened in May 1846 during the Mexican-American War. American forces, led by General Zachary Taylor, defended the fort on the Rio Grande against Mexican forces under General Mariano Arista. This was a crucial battle for control of the Texas-Mexico border.

Fort Texas was vital for American control in the region as it sat on the Rio Grande. The American forces successfully defended the fort, leading to their victory in the Battle of Palo Alto.

2. Battle of Monterrey (September 1846)

The Battle of Monterrey occurred from September 21 to September 24, 1846, in the Mexican-American War. General Zachary Taylor led American forces against General Pedro de Ampudia’s Mexican army in the strategically important city of Monterrey.

Monterrey was a key target for the American forces in northeastern Mexico. After intense urban combat, the Americans emerged victorious. 

3. Battle of Buena Vista (February 1847)

The Battle of Buena Vista, or Battle of Angostura, took place from February 22 to February 23, 1847. General Taylor led American forces against a larger Mexican force led by General Santa Anna in the mountainous terrain near Buena Vista, Mexico. Buena Vista was a critical test for the American army, facing a larger Mexican force.

Despite being outnumbered, the American forces held their ground and secured a strategic victory. This was not only because of the more well-trained American forces, but also because of the ill-treatment of Mexican soldiers by their officers. The latters were poorly trained and had little motivation to fight the Americans. 

4. Capture of Mexico City (September 1847)

The Capture of Mexico City marked the end of the Mexican-American War in September 1847. General Winfield Scott led American forces in a daring amphibious assault. He captured Mexico’s capital and prompted negotiations that resulted in the Treaty of Guadalupe Hidalgo.

General Scott’s campaign involved landing at Veracruz and advancing to Mexico City. The Battles of Cerro Gordo, Contreras, Churubusco, and Molino del Rey were key engagements leading to the capture of Mexico City and the end of the war.

Treaty of Guadalupe Hidalgo

  • Date Signed: February 2, 1848
  • Signing Location: Guadalupe Hidalgo, Mexico
  • Parties Involved: United States and Mexico

a) Terms and Consequences of  Mexican-American War: 

  • Mexico recognized the Rio Grande as the southern border of Texas
  • The United States acquired California, New Mexico, Arizona, Nevada, Utah, and parts of Colorado, Wyoming, Kansas, and Oklahoma
  • The U.S. agreed to pay Mexico $15 million
  • The U.S. took on the claims of American citizens against Mexico, amounting to $3.25 million

b) Impact on Territorial Boundaries

  • Established the modern southern border of the United States
  • U.S. territorial expansion significantly increased

c) Economic and Social Ramifications of Mexican-American War:

  1. The acquisition of new territories fueled westward migration
  2. Contributed to the California Gold Rush
  3. Laid the groundwork for the transcontinental railroad
  4. Heightened sectional tensions over the extension of slavery
  5. Ignited debates over the morality of the war within the U.S.
  6. Deepened anti-American sentiment in Mexico
  7. Impact on indigenous populations and their displacement

Legacy of the Mexican-American War

The Mexican-American War had a big impact on both the United States and Mexico, shaping their histories and how they relate to each other. A major result was that the United States gained a lot of new land. 

The Treaty of Guadalupe Hidalgo, signed in 1848, gave the U.S. places like California, New Mexico, Arizona, Nevada, Utah, and parts of Colorado, Wyoming, Kansas, and Oklahoma. This led to more people moving west, the Gold Rush in California, and the start of the transcontinental railroad.

The war also caused more problems in the United States over slavery. People argued about whether the new territories taken from Mexico would allow slavery or not. This made the disagreements between the Northern and Southern states even worse. 

Here are some additional impacts of the war. 

1. Impact on U.S.-Mexican Relations

The war really changed how the United States and Mexico got along. Mexico lost a lot of land, and this made Mexicans feel very upset and unfriendly toward Americans for many years. Taking Mexican land made the relationship between the two countries strained for a long time. Even today, problems over borders and tensions from the war affect how the U.S. and Mexico deal with immigration, trade, and sharing their cultures.

2. Debate over the Morality and Necessity of the War

People in the United States argued a lot about whether the Mexican-American War was right or needed. People like Abraham Lincoln and others in the Whig party said the war was wrong because it was an unfair way to get more land and expand slavery. 

Those against the war said President James K. Polk tricked the country into starting it. This argument about whether the war was right or wrong made the split between the Northern and Southern states even bigger, leading to the Civil War.

3. Consequences for Indigenous Populations

The war was really bad for the Native American people living in the new territories. The Treaty of Guadalupe Hidalgo didn’t protect the rights of Native Americans in the taken lands very well. As more Americans moved west, Native Americans lost their homes, faced violence, and had a hard time keeping their traditional way of life. 

The big influx of new settlers and changes in politics made things worse for Native American communities. This struggle for survival and loss of land still affects Native Americans today, causing problems with land rights, preserving their culture, and facing economic challenges.

Notable Figures during Mexican-American War

The Mexican American war favored the American because of the transformational leadership style adopted by some notable figures. Here is the list of these leaders who had the greatest impact on the results. 

1. James K. Polk

James K. Polk was 11th in the list of presidents of the USA. His term was from 1845 to 1849; he played a central role in the war. He not only advocated for territorial expansion, but also pursued an aggressive agenda that fueled many hostilities with the Mexicans. 

His administration’s policies, like annexing Texas and seeking additional Mexican territories, set the stage for the conflict. Polk’s leadership during this time defined how the U.S. approached the war and influenced decisions that guided its course.

2. Zachary Taylor

Zachary Taylor was the 12th president of the USA. He was a career military officer and a major commander during the war. He led the US forces in battles such as the Buena Vista and Palo Alto. 

Taylor gained recognition for his strategic skills and victories against Mexican forces. His successes in the field contributed to the overall momentum of the war and influenced subsequent military strategies.

3. Winfield Scott

Winfield Scott, a prominent military figure and general, played a vital role in the later stages of the Mexican-American War. Known for his military expertise, Scott led U.S. forces in the successful amphibious landing at Veracruz and marched on Mexico City. 

The capture of Mexico City marked a turning point in the war and significantly influenced the negotiation of the Treaty of Guadalupe Hidalgo, which ended the conflict.

4. Santa Anna

Antonio López de Santa Anna, a Mexican military leader and political figure, had a complex role during the Mexican-American War. Serving as Mexico’s president and military commander at various times, Santa Anna faced both successes and setbacks. 

His absence from the Battle of Buena Vista and subsequent return to power showcased the political and military dynamics within Mexico during this turbulent period.

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In conclusion, the Mexican-American War was a significant conflict in the mid-19th century that shaped the borders of the United States and Mexico. From the Siege of Fort Texas to the Capture of Mexico City, the war involved key battles and strategies. 

The outcome, marked by the Treaty of Guadalupe Hidalgo, established the Rio Grande as the southern border of Texas. This war played a crucial role in shaping the geopolitical landscape of North America.


What was the Mexican-American War?

The Mexican-American War occurred from 1846 to 1848, involving a conflict between the United States and Mexico. It started because of disagreements about the Texas-Mexico border.

Why did the Mexican-American War start?

The war began mainly because the United States took over Texas in 1845, upsetting Mexico as it saw this as an infringement on its independence. Disputes over borders and a wish for more land also played a role.

Who were the main figures in the Mexican-American War?

Important people included General Zachary Taylor and General Winfield Scott from the United States, and General Santa Anna from Mexico. They had significant roles in various battles and strategies during the war.

What were the key battles of the Mexican-American War?

Major battles comprised the Siege of Fort Texas, the Battle of Monterrey, the Battle of Buena Vista, and the Capture of Mexico City. These fights were crucial in deciding the war’s outcome.

How did the Mexican-American War conclude?

The war ended in 1848 with the Treaty of Guadalupe Hidalgo. This agreement set the Rio Grande as Texas’ southern border and gave a large part of Mexican land to the United States in return for compensation.

Oleksandra Mamchii

Working as a academic lead at Best Diplomats.

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