Causes of the Mexican-American War


The Mexican-American War was caused by several incidents that took place between the United States and Mexico, including the Texas Revolution, the Annexation of Texas, and a dispute over the border between Mexico and Texas. Further, instability in the Mexican Government and America's desire to fulfill its "Manifest Destiny" helped cause the war.

James K Polk, 11th President, Portrait

President James K. Polk. Image Source: National Portrait Gallery.

What caused the Mexican-American War?

The immediate cause of the Mexican-American War was a dispute over the border between the United States and Mexico over the border between Texas and Mexico. The United States considered the Rio Grande River to be the border, while Mexico insisted it was the Nueces River, roughly 100 miles to the north.

However, the war was the result of longstanding tension between Mexico and the United States, stemming from Mexico’s initial invitation of American settlers into Texas in the 1820s. That invitation, which was meant to boost Mexico’s economy and control of its frontier with the United States, eventually led to the Texas Revolution. Following the Revolution, Mexico refused to recognize the Republic of Texas, which led to the border dispute between Mexico and Texas — a dispute that would be inherited by the United States in 1845.

The United States also had grievances against Mexico dating back to the early 1830s, including claims of abuses against American vessels and citizens. Despite attempts to negotiate with the Mexican government, these grievances remained unresolved.

The desire for westward expansion, or “Manifest Destiny,” was a strong driving force behind American expansionism. Before the Mexican-American War, the United States had acquired the Louisiana Territory and Florida and was in a dispute with England over Oregon. 

The instability of the Mexican government between 1821 and 1846, which changed more than 10 times, contributed to Mexico’s inability to maintain a consistent foreign policy. It also made governing the vast territory of Mexico difficult, especially in places far from Mexico City, particularly California.

Further complicating the situation between Mexico and the United States were rumors that Great Britain was helping Mexico to slow down westward expansion.

The causes of the war are essentially the following:

  1. Establishment of the independent Republic of Texas.
  2. Dispute between Texas and Mexico over the border.
  3. United States Annexation of Texas.
  4. Instability and weakness of the Mexican Government.
  5. American expansion and Manifest Destiny.

Causes of the Mexican-American War — History and Overview

This overview of the Mexican-American War covers the causes of the war in chronological order, beginning with the aftermath of the Texas Revolution.

Mexico Refuses to Recognize the Treaties of Velasco

During the Texas Revolution (1836–1836), Mexico believed the United States encouraged the uprising to annex Texas. Following the Battle of San Jacinto (April 21, 1836), General Antonio López de Santa Anna agreed to the Treaties of Velasco with the people of Texas. The treaties identified the Rio Grande River as the southern border of Texas and recognized Texas as an independent state. At the time, Texas included parts of present-day New Mexico, Colorado, and Arkansas.

Battle of San Jacinto, Painting, McArdle
The Battle of San Jacinto by Henry Arthur McArdle. Image Source: Wikipedia.

Andrew Jackson Rejects Texas Annexation

However, the Mexican Government refused to approve the treaties or recognized the independent Republic of Texas, claiming Santa Anna had been coerced into agreeing. In October, the people of Texas, fearing Mexican aggression, sought the protection of the United States. They voted to join the United States and Texas officials tried to open negotiations with the administration of President Andrew Jackson. However, Jackson delayed negotiations for various reasons:

  1. 1836 was an election year and Jackson did not want the issue of annexation to affect the election
  2. Mexico told the United States that if it annexed Texas, it would be considered an Act of War.
  3. Slavery was legal in Texas and the addition of a pro-slavery territory would upset the balance between Free States and Slaves States in Congress.

Martin Van Buren Rejects Texas Annexation

In November 1836, Martin Van Buren was elected as the 8th President of the United States. He took office on March 4, 1837. A Texas official, Memucan Hunt Jr., presented Van Buren with the Texas proposal, but Van Buren rejected it in August. Resolutions for annexation were also introduced in the Senate and House of Representatives and were defeated.

John Tyler Becomes President of the United States

In 1838, Mirabeau B. Lamar was elected President of Texas. He was against annexation, and the offer to the United States was withdrawn in 1838. Two years later, William Henry Harrison defeated Van Buren in the U.S. Election of 1840. After he delivered his inaugural speech in cold, wet weather, Harrison developed pneumonia. He died at 12:30 a.m. on April 4, 1841, and was succeeded in office by Vice President John Tyler.

Following Tyler’s ascension to the Presidency, events started to take place that would lead to the eventual annexation of Texas.

  1. Sam Houston was elected President of Texas and favored annexation. Houston tried to approach the United States regarding annexation, but his effort was unsuccessful.
  2. In 1842, Texas extended its border West to the Pacific Ocean, which included portions of California and northern Mexico.
  3. By 1843, the United States was concerned about Great Britain’s interest in Texas. Great Britain did not want to annex Texas, but it did want to trade with Texas, as did other European nations. Tyler and other supporters of slavery were also concerned the British would try to emancipate slaves in Texas.
  4. 1844 was an election year. President Tyler believed the Annexation of Texas would aid him in his re-election effort.
President John Tyler, Portrait
John Tyler. Image Source: Wikipedia.

The Senate Rejects the Tyler-Texas Treaty

Tyler re-opened negotiations with Texas regarding annexation in October 1843, sending Secretary of State Abel P. Upshur of Virginia to meet with Isaac Van Zandt, the Texas minister to the United States. The United States and Texas agreed to a treaty, which would admit Texas into the Union as a territory. 

The “Tyler-Texas Treaty” was submitted to the Senate in April 1844. Secretary of State, John C. Calhoun, included a letter with the treaty, known as the “Packenham Letter,” that appealed to the pro-slavery faction. Despite Calhoun’s effort, the anti-slavery faction in Congress — Whigs and Northern Democrats — defeated the proposal on June 8, 1844.

Tyler looked for an alternative to gaining approval for adding Texas to the Union, however, the Annexation of Texas became a key issue for both Whigs and Democrats in the Election of 1844.

The Election of 1844

Henry Clay, the Whig candidate, did not fully endorse annexation. He was not opposed to it, but he was cautious and expressed concerns over the risks. Clay was concerned annexation would lead to war with Mexico and increase the sectional divide over slavery.

For the Democrats, the path to electing a nominee was complicated. Tyler dropped out of the race, and Martin Van Buren, who was considered the favorite, opposed annexation. A third candidate, James K. Polk from Tennessee, fully endorsed annexation. Polk, the “dark horse candidate,” won the nomination with support from Andrew Jackson — and then narrowly defeated Clay in the election.

Congress Approves Annexation

When the members of Congress returned to Washington, D.C. in December, President Tyler resumed his effort to gain approval for the Tyler-Texas Treaty. He proposed the treaty be introduced as a joint resolution, which would only require a majority vote in both the Senate and the House. With Polk’s support, the joint resolution was passed and signed by President Tyler on March 1, 1845 — three days before Polk was sworn in as President.

Mexico responded to the passage of the joint resolution by recalling its minister to the United States, Juan Almonte, from Washington. Almonte requested his passport on March 6, effectively cutting off diplomatic relations between Mexico and the United States. However, the President of Mexico, José Joaquin Herrera, did not want to pursue war and looked for other ways to block annexation.

April 1845 — Taylor Takes Command

Soon after Polk took office, he started to prepare for the possibility of hostilities with Mexico. On April 23, 1845, General Zachary Taylor was placed in command of the 1st Military District. Four days later, on April 27, he received orders to assemble a “Corps of Observation” and prepare to move into Texas in case of a Mexican invasion.

President Zachary Taylor, Official Portrait, 1849, Bush
Zachary Taylor. Image Source: White House Historical Society.

Frémont’s Expedition to California

While Polk was preparing for hostilities along the Texas border, he also set in motion a series of events that would lead to clashes with Mexican officials in California. 

Responding to rumors that Great Britain was working with Mexico in California, Polk, Thomas Hart Benton, a Senator from Missouri, and George Bancroft, Secretary of the Navy, Polk planned to send a military expedition to California. 

Publicly, the purpose of the mission would be led by Benton’s son, John C. Frémont. Privately, it may have been to convince Americans living in Mexico to rise up and revolt against the Mexican Government.

June 1845 — Taylor Moves to the Nueces River

After Congress approved annexation, Texas needed to vote on the issue, which was set to take place on July 4. Fearing a Mexican invasion of Texas, Polk ordered Taylor to move out of Fort Jessup in Louisiana and move into Texas, somewhere “on or near” the Rio Grande River on June 25, 1845.

A month later, on July 25, Taylor started to move out of New Orleans, when nearly all of his 1,500 men boarded steamboats and sailed for Texas. His dragoons traveled overland and made their way to San Antonio and then to Corpus Christie. It was there, where the Nueces River flows into the Gulf of Mexico, that Taylor established his camp. 

At Corpus Christie, Taylor trained and drilled his “Army of Observation,” which grew to approximately 4,000 men. They were joined by volunteers, including a company of Texas Rangers who operated as scouts, keeping an eye out for trouble.

Mexican American War, Army of Observation at Corpus Christi, 1845
The Army of Observation Camp at Corpus Christi, Texas. Image Source: Library of Congress.

July 1845 — Manifest Destiny

On July 1, newspaper editor John O’Sullivan coined the phrase “Manifest Destiny.” Arguing in favor of annexing Texas, O’Sullivan said, “Annex the Republic of Texas, not only because Texas desired this, but because it was our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions.”

December 1845 – Texas Joins the Union

Texas voted in favor of annexation on July 4, 1845, and then drafted a state constitution, which was approved in a popular vote of the people in October. Congress approved the constitution on December 29, 1845, and Texas officially entered the Union. However, the formal transfer of power was done in a ceremony that took place on February 19, 1846. At the ceremony, Texas President Anson Jones declared “The final act in this great drama is now performed; the Republic of Texas is no more,” and James Pinckney Henderson assumed the office of Governor of Texas.

Dispute with Mexico Over the Nueces Strip

When the United States annexed Texas, American officials hoped it would give them an opportunity to discuss the border with Mexico. However, annexation only heightened tension, especially over the “Nueces Strip” — the disputed territory between the Nueces River and the Rio Grande River. Mexico insisted the Nueces River was its northern border, while Texas believed it was roughly 100 miles further south, at the Rio Grande River. 

Mexican American War, Nueces Strip, Illustration
This map shows the disputed territory between the United States and Mexico. Image Source: U.S. Army Center of Military History.

November 1845 — Polk Sends John Slidell to Mexico

Meanwhile, in the fall of 1845, John Black, the U.S. consul in Mexico, met with the Mexican Minister of Foreign Affairs, Manuel de la Peña y Pena. The meeting left Black with the impression that the Mexican Government was open to discussing several issues, including the border. Black notified the State Department that Mexico was ready to restore diplomatic relations

On November 10, President Polk selected John Slidell, a member of the House of Representatives from Louisiana, to go to Mexico City to negotiate. Slidell, who was fluent in Spanish, was instructed to offer $5 million for the New Mexico Territory, $20 million for California, and $5 million for the disputed territory in the Nueces Strip. If Mexico agreed, the United States would stretch from sea to sea, and the Rio Grande River would be the southern border of the United States, between Texas and Mexico.

John Slidell, Louisiana Congressman, Photograph
John Slidell. Image Source: New York Public Library Digital Collections.

Slidell Arrives in Mexico

Slidell arrived in Mexico on December 6. Soon after, he found that the Mexican Government refused to meet with him. It seems that Black misunderstood what Manuel de la Peña y Pena, the Mexican Minister of Foreign Affairs, was actually willing to discuss with the United States, and who he would discuss it with. It turned out Mexico was only open to talking about the border issue, but not with a special minister, who was powerless to make any decisions.

December 1845 — President Hererra Ousted in Mexico

Any hope Slidell had of success was eliminated when General Mariano Paredes y Arrillaga led a successful coup. On December 30, President Herrera resigned and Paredes and his military forces marched into Mexico City on January 2. The next day, Paredes became the new President of Mexico, and he had no intention of negotiating with the United States.

Slidell sent a message to Washington, informing Polk of the failure of his mission. In mid-January, Secretary of State James Buchanan told Slidell to return to Washington unless Mexico agreed to meet with him and recognize his credentials. 

January 1846 — Polk Orders Taylor to the Rio Grande River

Around the same time, on January 13, Polk sent orders to Taylor, instructing him to cross the Nueces River, into the Nueces Strip, and move down to the Rio Grande. 

Polk’s orders reached Taylor in February. In March, he moved south to the north side of the Rio Grande and established a camp for his army near present-day Brownsville, Texas, and also had his men build Fort Texas. The Americans were across from the town of Matamoros, Mexico, and the Mexicans considered it a serious threat. 

In response, Mexican General Mariano Arista demanded that Taylor withdraw from his camp and move back north of the Nueces River. However, Taylor refused the request and Arista marched his army toward Fort Texas. Taylor responded to the threat by asking the U.S. Navy to blockade the mouth of the Rio Grande River.

Meanwhile, Mexico continued to refuse to meet with Slidell, so he asked for his passports and returned to Washington.

April 1846 — The Thornton Affair

On April 24, Taylor was informed that Mexican troops had crossed the Rio Grande and were headed toward Fort Texas. Taylor responded by sending Captain Seth Thornton and a small force to scout the area, see if they could find the Mexican force, and gather intelligence on it. 

The next day, Thornton and his men were at Rancho de Carricitos, just east of present-day Bluetown, Texas. The Mexicans surrounded them at the ranch and attacked, killing at least 11 Americans and wounding 6. 46 men, including Captain William J. Hardee, were taken as prisoners to Matamoros in the aftermath of the Thornton Affair.

Thornton Affair, 1836, Illustration
This illustration depicts the Thornton Affair. Image Source: Pictorial History of the Mexican War (1850),

Mexican Forces Lay Siege to Fort Texas

On May 1, 1846, General Arista crossed the Rio Grande to place his forces between Taylor’s army at Fort Texas and the American supply base at Point Isabel. Taylor left to secure his supply line and placed Major Jacob Brown in command of Fort Texas. Two days later, Mexican forces led by General Pedro de Ampudia opened fire on the fort.

Taylor Engages Arista at Palo Alto and Resaca de la Palma

On May 7, Taylor left Point Isabel and started his march back to Fort Texas. He had around 2,300 men and more than 200 wagons loaded with supplies. 

On the way back to Fort Texas, Arista engaged Taylor at the Battle of Palo Alto (May 8). The Americans won the battle, and the Mexicans withdrew that night. Taylor followed Arista and they engaged each other the next day at the Battle of Resaca de la Palma (May 9). Once again, the Americans defeated the Mexicans, sending them back over the Rio Grande into Mexico. With Arista’s defeat, General Ampudia was forced to lift the Siege of Fort Texas and fall back with the rest of the Mexican forces. 

Battle of Resaca de la Palma, 1846, May's Charge
This painting depicts Charles May leading his dragoons at the Battle of Resaca de la Palma. Image Source: Wikipedia.

Polk Prepares to Ask for a Declaration of War

Slidell arrived in Washington and met with President Polk on May 8. Polk followed that by meeting with his cabinet on May 9. During that meeting, Polk suggested Mexico’s refusal to meet with Slidell was enough to ask Congress for a Declaration of War against Mexico. Secretary of the Navy George Bancroft was the only member of the cabinet who disagreed with Polk. Bancroft suggested it would be better to wait for Mexico to make a move that warranted the use of military force. Despite Bancroft’s warning, Polk started working on a war message to take to Congress and ask for a Declaration of War.

Polk Learns of the Thornton Affair

Around 6:00 that night, Polk received a message from General Taylor, informing him that Mexican forces had attacked an American patrol on the north side of the Rio Grande River. What Polk did not know yet was that Taylor had engaged the Mexican Army on May 8 at Palo Alto and earlier that day at Resaca de la Palma. Taylor won both battles, which forced the Mexicans to retreat over the Rio Grande River, south to Matamoros, Mexico. It also ended the Siege of Fort Texas. With the knowledge that hostilities were underway, Polk continued to craft his war message.

President James K. Polk, Cabinet Members, 1846
President James K. Polk, Cabinet Members, 1846. Image Source: White House Historical Society.

This photograph was taken by John Plumbe, Jr. in 1846. It shows President James K. Polk and his cabinet assembled in the State Dining Room. It is the earliest photograph of the interior of the White House and the first photograph of a U.S. President and his advisors.

Seated, left to right: Attorney General John Y. Mason, Secretary of War William Marcy, President Polk, Secretary of the Treasury Robert J. Walker; standing: Postmaster General Cave Johnson (center); Secretary of the Navy George Bancroft.

Citation Information

The following information is provided for citations, including APA Style, Chicago Style, and MLA Style.

  • Article Title Causes of the Mexican-American War
  • Date 1836–1846
  • Author
  • Keywords Mexican-American War Causes, Texas Revolution, Manifest Destiny, Andrew Jackson, Martin Van Buren, John Tyler, James K. Polk, Election of 1844, Annexation of Texas, Zachary Taylor, Army of Occupation, Nueces Strip, John Slidell
  • Website Name American History Central
  • Access Date May 27, 2024
  • Publisher R.Squared Communications, LLC
  • Original Published Date
  • Date of Last Update September 12, 2023