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5.3 The Mexican–American War

6 min readjanuary 11, 2023

Caleb Lagerwey

Caleb Lagerwey

Dalia Savy

Dalia Savy

Caleb Lagerwey

Caleb Lagerwey

Dalia Savy

Dalia Savy

goes south...but not too far south.

The Mexican-American War was a conflict between the United States and Mexico that took place between 1846 and 1848. The war was sparked by a dispute over the by the United States and a long-standing dispute over the border between Texas and Mexico.

Context to the Mexican-American War

A preface to the was the (1835-1836), a military conflict between the (which was established by American settlers) and Mexico. The war ended with the decisive victory of the Texan army, led by , at the in April 1836. This resulted in the creation of the , which was an independent nation for several years before it was annexed by the United States in 1845.

This war involved and other famous battles like Goliad and San Jacinto that were part of period three, but let's quickly discuss them.

is a key and iconic battle that took place during the Texan Revolution, where a small Texan army, mostly made of American settlers, was besieged by a larger Mexican army for 13 days. The defenders, which included famous figures like , , and , were all killed by the Mexican army. The battle, despite the Texan defeat, became a symbol of Texan resistance and sacrifice and helped to inspire the Texan army to victory at San Jacinto.

The was a series of events that took place in March 1836 and it was led by . He ordered his troops to execute more than 400 Texan prisoners, including the Commander and soldiers captured at the , who had been promised quarter, this event became known as the . This event outraged many Texans and led them to fight more fiercely, leading to the defeat of the Mexican Army at San Jacinto.

The by the United States in 1845 was a major point of contention between the two countries, and it was one of the main causes of the , which began the following year.

Causes of the Mexican-American War

Here are the main causes of the , in bullet form:

  • Dispute over the border between Texas and Mexico. The U.S. claimed that the border was the , while Mexico insisted that it was the .

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-lFCUzpNRGgd9.png?alt=media&token=d9356a4c-4248-4431-abaa-6d7a9807311d

Image Courtesy of Texas Higher Education Coordinating Board

  • by the United States in 1845. Mexico had never recognized the independence of Texas and considered it to be a part of its territory.

  • Expansionist policies of the U.S. government. Pro-expansionists were largely Democrats and had long eyed Mexico and other sub-tropical regions for their ability to grow crops that could lead to the expansion of slavery and the associated Southern way of life. Mexico seemed like a prime target. was elected on a pro-expansion platform and sought to expand U.S. territory through Mexico.

  • Economic and strategic interests. The U.S sought access to ports in Mexico and the California territory, with its vast resources and growing population.

In an effort to protect its claimed territory, the U.S. government under ordered U.S. troops, under the command of , to occupy land along the . This move was seen as a provocation by the Mexican government because of the dispute over the border, and the Mexican government responded by sending its own troops to the area.

The United States won after two years of battles and negotiations, and you can view them briefly on the map below. You do not have to know the specifics of the war itself, just the causes and effects of it!

The Treaty of Guadalupe Hidalgo

The , signed on February 2, 1848, was a peace treaty between the United States and Mexico that officially ended the .

Under the terms of the treaty, Mexico ceded a large portion of its territory to the United States, which we'll discuss below. Mexico was also compensated with $15 million and the U.S. assumed responsibility for the claims of American citizens against Mexico.

The treaty also included several provisions related to the rights of Mexican citizens living in the ceded territory. These provisions guaranteed them the right to remain in the territory and become U.S. citizens, to continue to practice their Catholic religion, and to retain their property rights. However, these rights were not fully respected and enforced, and in the following years, many Mexicans living in the ceded territory faced discrimination, violence, and the loss of their land.

Effects of the Mexican-American War

The was an important event as it serves as a link between and the : it is a great example of how Westward expansion led to increasingly bitter and divisive debates over slavery in new territories.

The had a number of significant effects on both the United States and Mexico. Some of the most notable effects include:

  • Territorial changes: Mexico ceded a large portion of its territory to the United States, including present-day California, Nevada, Utah, Arizona, New Mexico, and parts of Colorado, Wyoming, Kansas, and Oklahoma. This loss of territory had a profound impact on Mexico and its people, and it is still a sensitive topic in Mexico to this day. The land that was lost to the United States was known as the .

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-qedDaIf8shNW.png?alt=media&token=248158e2-f920-4c45-bafb-e7873c575cbf

The Mexican Cession, shown in red, and the later Gadsden Purchase, shown in yellow; Image Courtesy of Matthew Trump on Wikimedia

  • Economic and political effects: The war had an impact on the economy of both countries. The U.S. acquired new territory and resources, and the war led to economic growth and expansion in the West. On the other hand, Mexico experienced a loss of territory, resources, and population and significant debt. The war also led to the rise of political factions in Mexico and contributed to the instability of the country's political landscape in the decades that followed.

  • Slavery and race relations: The war had an impact on the issue of slavery in the United States. The acquisition of new territory and the question of whether or not to allow slavery in those territories became a major point of contention between the Northern and Southern states, which ultimately helped to fuel the growing divide between the two regions and contributed to the eventual outbreak of the American .

  • Impact on Native Americans: The war and the subsequent U.S. expansion led to the displacement and forced relocation of many indigenous people, which had a significant impact on their communities and cultures.

  • Relations with Mexico: The war left bitterness and mistrust between the two countries, the U.S. and the treatment of Mexico during the war caused many Mexicans to view the U.S. as an imperialistic and aggressive neighbor.

The Wilmot Proviso

The was an unsuccessful addition to a bill to fund the US army during the war. It argued for a complete ban on slavery in captured territories, which, of course, only made tensions worse. It passed the House of Representatives—hello, Northern population advantage!—but failed in the Senate.

This showcased the necessity for Southerners to keep the balance of states in the Senate as their population totals fell behind those of the North: the Senate, which is equal representation for each state regardless of population, was the safeguard for the South’s interests as the North continued to add to its population advantage in the House and Electoral College.

Key Terms to Review (23)

Annexation of Texas

: The annexation of Texas refers to when Texas joined the United States as its 28th state in 1845. This followed nearly a decade after it won independence from Mexico and existed as an independent Republic.

Battle of Coleto

: The Battle of Coleto, fought in March 1836 during the Texas Revolution, was a confrontation between Texian rebels and Mexican forces. The Texians were surrounded and forced to surrender, leading to the infamous Goliad Massacre.

Battle of San Jacinto

: The Battle of San Jacinto was the decisive battle in 1836 where Texan forces under Sam Houston defeated Mexican troops, leading directly to Texan independence.

Civil War

: The Civil War was a major conflict fought between northern states (known as the Union) and southern states (known as Confederacy) that seceded from the United States over issues including slavery and states' rights from 1861-1865.

Davy Crockett

: Davy Crockett was a 19th-century American folk hero, frontiersman, soldier, and politician. He is commonly referred to in popular culture by the epithet "King of the Wild Frontier". He represented Tennessee in the U.S. House of Representatives and served in the Texas Revolution.

General Santa Anna

: Antonio López de Santa Anna was a Mexican politician and general who fought to defend royalist New Spain and then for Mexican independence. He played key roles in both sides - he helped overthrow Mexico's Spanish colonial government, and then he tried to suppress Texas' fight for independence.

General Zachary Taylor

: Zachary Taylor was a U.S. military leader who served as the 12th president of United States from 1849 until his death in 1850. He became a national hero due to his victories in Mexican-American War.

Goliad Campaign

: The Goliad Campaign was a series of 19th-century military events that took place in Texas in 1836, during the Texas Revolution. It ended with the infamous Goliad Massacre.

Goliad Massacre

: The Goliad Massacre was an event that occurred during the Texas Revolution in 1836, following the Battle of Coleto Creek. Over 400 Texian soldiers were executed by Mexican forces under orders from General Santa Anna.

James Bowie

: James Bowie was a 19th-century American pioneer who played a prominent role in the Texas Revolution, culminating in his death at the Battle of Alamo. He became famous for his knife-fighting skills and his eponymous "Bowie knife".

Manifest Destiny

: Manifest Destiny was a 19th-century belief that the United States was destined and divinely ordained to expand across the North American continent, from the Atlantic seaboard to the Pacific Ocean.

Mexican Cession

: The Mexican Cession is a term for the region in present-day southwestern United States that Mexico ceded to the U.S. in 1848 under the Treaty of Guadalupe Hidalgo following the Mexican-American War.

Mexican-American War

: The Mexican-American War (1846-1848) was a conflict between Mexico and America that followed America's annexation of Texas, which Mexico still considered its territory despite an independent Texan Republic existing since 1836.

Nueces River

: The Nueces River is a river located in the U.S. state of Texas, which was historically significant as the disputed border between Texas and Mexico prior to the Mexican-American War.

President James K. Polk

: James K. Polk served as the 11th president of United States from 1845-1849. He is known for his policy of westward expansion which led to U.S acquiring California and much territory in Southwest after winning Mexican-American war.

Republic of Texas

: The Republic of Texas was an independent sovereign country in North America that existed from March 2, 1836, to February 19, 1846. It was formed as a separate nation after gaining independence from Mexico during the Texas War for Independence.

Rio Grande

: The Rio Grande is a river that forms part of the border between Mexico and Texas. It was central to boundary disputes during the Mexican-American War.

Sam Houston

: Sam Houston was a key figure in the history of Texas, serving as its first president when it became an independent republic in 1836 and later as its governor when it joined the United States. He played a crucial role in Texas' fight for independence from Mexico.

Texas War of Independence

: The Texas Revolution or Texas War for Independence was fought from October 2nd,1835 to April 21st ,1836 between Mexico and rebellious colonists from the U.S living in Texas who wanted independence.

The Alamo

: The Alamo was a mission in San Antonio, Texas, that was the site of a famous battle during the Texas Revolution in 1836 where Texan defenders were overrun by Mexican forces.

Treaty of Guadalupe Hidalgo

: The Treaty of Guadalupe Hidalgo was a peace agreement signed in 1848 that ended the Mexican-American War. It led to the United States acquiring over half of Mexico's territory, including present-day California, Nevada, Utah, Arizona, New Mexico and parts of Colorado, Wyoming, Kansas and Oklahoma.

William Barret Travis

: William Barret Travis was a 19th-century American lawyer and soldier. He is best known as the commander of the Texian forces at the Battle of Alamo, where he died.

Wilmot Proviso

: The Wilmot Proviso was an unsuccessful proposal in 1846 that aimed at banning slavery in any territory gained from Mexico as a result of the Mexican-American War.

5.3 The Mexican–American War

6 min readjanuary 11, 2023

Caleb Lagerwey

Caleb Lagerwey

Dalia Savy

Dalia Savy

Caleb Lagerwey

Caleb Lagerwey

Dalia Savy

Dalia Savy

goes south...but not too far south.

The Mexican-American War was a conflict between the United States and Mexico that took place between 1846 and 1848. The war was sparked by a dispute over the by the United States and a long-standing dispute over the border between Texas and Mexico.

Context to the Mexican-American War

A preface to the was the (1835-1836), a military conflict between the (which was established by American settlers) and Mexico. The war ended with the decisive victory of the Texan army, led by , at the in April 1836. This resulted in the creation of the , which was an independent nation for several years before it was annexed by the United States in 1845.

This war involved and other famous battles like Goliad and San Jacinto that were part of period three, but let's quickly discuss them.

is a key and iconic battle that took place during the Texan Revolution, where a small Texan army, mostly made of American settlers, was besieged by a larger Mexican army for 13 days. The defenders, which included famous figures like , , and , were all killed by the Mexican army. The battle, despite the Texan defeat, became a symbol of Texan resistance and sacrifice and helped to inspire the Texan army to victory at San Jacinto.

The was a series of events that took place in March 1836 and it was led by . He ordered his troops to execute more than 400 Texan prisoners, including the Commander and soldiers captured at the , who had been promised quarter, this event became known as the . This event outraged many Texans and led them to fight more fiercely, leading to the defeat of the Mexican Army at San Jacinto.

The by the United States in 1845 was a major point of contention between the two countries, and it was one of the main causes of the , which began the following year.

Causes of the Mexican-American War

Here are the main causes of the , in bullet form:

  • Dispute over the border between Texas and Mexico. The U.S. claimed that the border was the , while Mexico insisted that it was the .

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-lFCUzpNRGgd9.png?alt=media&token=d9356a4c-4248-4431-abaa-6d7a9807311d

Image Courtesy of Texas Higher Education Coordinating Board

  • by the United States in 1845. Mexico had never recognized the independence of Texas and considered it to be a part of its territory.

  • Expansionist policies of the U.S. government. Pro-expansionists were largely Democrats and had long eyed Mexico and other sub-tropical regions for their ability to grow crops that could lead to the expansion of slavery and the associated Southern way of life. Mexico seemed like a prime target. was elected on a pro-expansion platform and sought to expand U.S. territory through Mexico.

  • Economic and strategic interests. The U.S sought access to ports in Mexico and the California territory, with its vast resources and growing population.

In an effort to protect its claimed territory, the U.S. government under ordered U.S. troops, under the command of , to occupy land along the . This move was seen as a provocation by the Mexican government because of the dispute over the border, and the Mexican government responded by sending its own troops to the area.

The United States won after two years of battles and negotiations, and you can view them briefly on the map below. You do not have to know the specifics of the war itself, just the causes and effects of it!

The Treaty of Guadalupe Hidalgo

The , signed on February 2, 1848, was a peace treaty between the United States and Mexico that officially ended the .

Under the terms of the treaty, Mexico ceded a large portion of its territory to the United States, which we'll discuss below. Mexico was also compensated with $15 million and the U.S. assumed responsibility for the claims of American citizens against Mexico.

The treaty also included several provisions related to the rights of Mexican citizens living in the ceded territory. These provisions guaranteed them the right to remain in the territory and become U.S. citizens, to continue to practice their Catholic religion, and to retain their property rights. However, these rights were not fully respected and enforced, and in the following years, many Mexicans living in the ceded territory faced discrimination, violence, and the loss of their land.

Effects of the Mexican-American War

The was an important event as it serves as a link between and the : it is a great example of how Westward expansion led to increasingly bitter and divisive debates over slavery in new territories.

The had a number of significant effects on both the United States and Mexico. Some of the most notable effects include:

  • Territorial changes: Mexico ceded a large portion of its territory to the United States, including present-day California, Nevada, Utah, Arizona, New Mexico, and parts of Colorado, Wyoming, Kansas, and Oklahoma. This loss of territory had a profound impact on Mexico and its people, and it is still a sensitive topic in Mexico to this day. The land that was lost to the United States was known as the .

https://firebasestorage.googleapis.com/v0/b/fiveable-92889.appspot.com/o/images%2F-qedDaIf8shNW.png?alt=media&token=248158e2-f920-4c45-bafb-e7873c575cbf

The Mexican Cession, shown in red, and the later Gadsden Purchase, shown in yellow; Image Courtesy of Matthew Trump on Wikimedia

  • Economic and political effects: The war had an impact on the economy of both countries. The U.S. acquired new territory and resources, and the war led to economic growth and expansion in the West. On the other hand, Mexico experienced a loss of territory, resources, and population and significant debt. The war also led to the rise of political factions in Mexico and contributed to the instability of the country's political landscape in the decades that followed.

  • Slavery and race relations: The war had an impact on the issue of slavery in the United States. The acquisition of new territory and the question of whether or not to allow slavery in those territories became a major point of contention between the Northern and Southern states, which ultimately helped to fuel the growing divide between the two regions and contributed to the eventual outbreak of the American .

  • Impact on Native Americans: The war and the subsequent U.S. expansion led to the displacement and forced relocation of many indigenous people, which had a significant impact on their communities and cultures.

  • Relations with Mexico: The war left bitterness and mistrust between the two countries, the U.S. and the treatment of Mexico during the war caused many Mexicans to view the U.S. as an imperialistic and aggressive neighbor.

The Wilmot Proviso

The was an unsuccessful addition to a bill to fund the US army during the war. It argued for a complete ban on slavery in captured territories, which, of course, only made tensions worse. It passed the House of Representatives—hello, Northern population advantage!—but failed in the Senate.

This showcased the necessity for Southerners to keep the balance of states in the Senate as their population totals fell behind those of the North: the Senate, which is equal representation for each state regardless of population, was the safeguard for the South’s interests as the North continued to add to its population advantage in the House and Electoral College.

Key Terms to Review (23)

Annexation of Texas

: The annexation of Texas refers to when Texas joined the United States as its 28th state in 1845. This followed nearly a decade after it won independence from Mexico and existed as an independent Republic.

Battle of Coleto

: The Battle of Coleto, fought in March 1836 during the Texas Revolution, was a confrontation between Texian rebels and Mexican forces. The Texians were surrounded and forced to surrender, leading to the infamous Goliad Massacre.

Battle of San Jacinto

: The Battle of San Jacinto was the decisive battle in 1836 where Texan forces under Sam Houston defeated Mexican troops, leading directly to Texan independence.

Civil War

: The Civil War was a major conflict fought between northern states (known as the Union) and southern states (known as Confederacy) that seceded from the United States over issues including slavery and states' rights from 1861-1865.

Davy Crockett

: Davy Crockett was a 19th-century American folk hero, frontiersman, soldier, and politician. He is commonly referred to in popular culture by the epithet "King of the Wild Frontier". He represented Tennessee in the U.S. House of Representatives and served in the Texas Revolution.

General Santa Anna

: Antonio López de Santa Anna was a Mexican politician and general who fought to defend royalist New Spain and then for Mexican independence. He played key roles in both sides - he helped overthrow Mexico's Spanish colonial government, and then he tried to suppress Texas' fight for independence.

General Zachary Taylor

: Zachary Taylor was a U.S. military leader who served as the 12th president of United States from 1849 until his death in 1850. He became a national hero due to his victories in Mexican-American War.

Goliad Campaign

: The Goliad Campaign was a series of 19th-century military events that took place in Texas in 1836, during the Texas Revolution. It ended with the infamous Goliad Massacre.

Goliad Massacre

: The Goliad Massacre was an event that occurred during the Texas Revolution in 1836, following the Battle of Coleto Creek. Over 400 Texian soldiers were executed by Mexican forces under orders from General Santa Anna.

James Bowie

: James Bowie was a 19th-century American pioneer who played a prominent role in the Texas Revolution, culminating in his death at the Battle of Alamo. He became famous for his knife-fighting skills and his eponymous "Bowie knife".

Manifest Destiny

: Manifest Destiny was a 19th-century belief that the United States was destined and divinely ordained to expand across the North American continent, from the Atlantic seaboard to the Pacific Ocean.

Mexican Cession

: The Mexican Cession is a term for the region in present-day southwestern United States that Mexico ceded to the U.S. in 1848 under the Treaty of Guadalupe Hidalgo following the Mexican-American War.

Mexican-American War

: The Mexican-American War (1846-1848) was a conflict between Mexico and America that followed America's annexation of Texas, which Mexico still considered its territory despite an independent Texan Republic existing since 1836.

Nueces River

: The Nueces River is a river located in the U.S. state of Texas, which was historically significant as the disputed border between Texas and Mexico prior to the Mexican-American War.

President James K. Polk

: James K. Polk served as the 11th president of United States from 1845-1849. He is known for his policy of westward expansion which led to U.S acquiring California and much territory in Southwest after winning Mexican-American war.

Republic of Texas

: The Republic of Texas was an independent sovereign country in North America that existed from March 2, 1836, to February 19, 1846. It was formed as a separate nation after gaining independence from Mexico during the Texas War for Independence.

Rio Grande

: The Rio Grande is a river that forms part of the border between Mexico and Texas. It was central to boundary disputes during the Mexican-American War.

Sam Houston

: Sam Houston was a key figure in the history of Texas, serving as its first president when it became an independent republic in 1836 and later as its governor when it joined the United States. He played a crucial role in Texas' fight for independence from Mexico.

Texas War of Independence

: The Texas Revolution or Texas War for Independence was fought from October 2nd,1835 to April 21st ,1836 between Mexico and rebellious colonists from the U.S living in Texas who wanted independence.

The Alamo

: The Alamo was a mission in San Antonio, Texas, that was the site of a famous battle during the Texas Revolution in 1836 where Texan defenders were overrun by Mexican forces.

Treaty of Guadalupe Hidalgo

: The Treaty of Guadalupe Hidalgo was a peace agreement signed in 1848 that ended the Mexican-American War. It led to the United States acquiring over half of Mexico's territory, including present-day California, Nevada, Utah, Arizona, New Mexico and parts of Colorado, Wyoming, Kansas and Oklahoma.

William Barret Travis

: William Barret Travis was a 19th-century American lawyer and soldier. He is best known as the commander of the Texian forces at the Battle of Alamo, where he died.

Wilmot Proviso

: The Wilmot Proviso was an unsuccessful proposal in 1846 that aimed at banning slavery in any territory gained from Mexico as a result of the Mexican-American War.


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.


© 2024 Fiveable Inc. All rights reserved.

AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.