Why did the United States start the Mexican American War

The Mexican-American War began in 1846 when the United States claimed that the Mexican army had crossed over into their territory and opened fire on American soldiers. The dispute centered around a region of land between Texas, Mexico and the Pacific Ocean. The United States wanted to acquire this part of Mexico, but Mexico disagreed, claiming that their border was further south along the Nueces River. When the Mexican army opened fire on American soldiers at a disputed border area near Texas, it sparked off a series of events which led to the declaration of war between both countries.

Polk looked to Expand United States territory

The United States had been eyeing Mexican territory for some time, and President James Polk had been looking for an opportunity to annex it. He saw the incident as an excuse to declare war and take advantage of an unprepared Mexico. Congress declared war on Mexico in May 1846, with most of the legislators backing Polk's expansionist political agenda of Manifest Destiny: the belief that American settlers should extend their control over the American continent all the way up to the Pacific Ocean. The Mexican-American War was fought primarily over disputed land that both countries claimed as their own, including Texas which had declared itself independent from Mexico a few years before.

In addition to this territorial dispute, Polk was also motivated by his desire to establish America's presence in Central America and expand its influence across Latin America. Through annexation he could also strengthen his party’s control over Congress while increasing economic development through new markets and resources. The expansionist ambitions of President James Polk ultimately led to a two year long conflict which resulted in Mexico ceding more than half its territory to the United States at its conclusion, in what became known as The Treaty of Guadalupe Hidalgo in 1848.

The spark that ignited the war was a dispute between the United States and Mexico over the border between the United States and Mexico. Polk argued that Mexican troops had crossed into American territory and initiated attacks against American soldiers. In order to gain public support for his position, Polk used news reports to paint Mexico as the aggressor in an effort to galvanize public opinion against them. Polk's claim was dubious at best.

The United States invaded Mexico

General Zachary Taylor was sent with a large force of troops near the disputed region in an attempt to maintain peace, but tensions continued to escalate between both sides as Mexican forces moved closer to him. Eventually on April 25th 1846, President James Polk declared war on Mexico after their refusal of his offer of payment for any disputed territory or damages caused by previous skirmishes.

The Mexican government viewed this diplomatic mission, led by the Democratic politician, John Slidell as an affront to their nation's sovereignty. This prompted military incitements from both sides and led to quick American victories in many battles. Despite the swiftness of these successes, the war continued on for two years until it was officially ended in 1848. Politician James Polk is often credited with starting the war due to his failed attempt at diplomacy and Slidell's mission being viewed by Mexico as an insult.

On May 11, 1846, President Polk gave his war message to Congress in which he stated that Mexico had shed American blood on American soil. This was in reference to the Mexican troops who had killed some of the American troops at the Rio Grande. He further argued that the refusal by Mexico to accept America's right to annex Texas and its claim to the Oregon territory justified declaring war. Although Polk's actions were initially supported by many Americans, critics later charged that Polk had deliberately provoked a war with Mexico for political gain and territorial expansion.

US Congressman John Slidell sent to Mexico to buy its territory

In 1845, President James Polk dispatched Congressman John Slidell to Mexico City in an effort to purchase Mexican territory and resolve a border dispute. The Mexican government refused to negotiate, however, and Polk encouraged Thomas Benton to introduce a resolution in the US Senate declaring that war existed between the two countries. In 1846, American forces moved into disputed Mexican territory along the Rio Grande River and were attacked by Mexican soldiers. In response, Polk asked Congress for a declaration of war against Mexico. He argued that Mexico had invaded American soil first and that it was necessary for the US to defend its rights and interests in Texas as well as pursue its ambitions for expansion of the Union.

In addition to disputes over Texas's independence from Mexico, tensions between America and Mexico had been mounting since settlers began moving into Alta California after gold was discovered there in 1848. The American consul reported numerous complaints from Americans regarding mistreatment by Mexicans while attempting to traverse through or reside in Mexican settlements; this further encouraged President James Polk's decision to go forward with war against Mexico despite warnings from his cabinet members not do so.

The President and his expansionist Democrats were not content with only the annexation of Texas and felt that they had expansive border claims to New Mexico, Alta California, South Texas and other Mexican territories. They wanted complete annexation of all these regions. A border dispute broke out between the two countries after the annexation of Texas; this dispute was further intensified by Polk's request that Mexico relinquish its hold on Nuevo Mexico. This led to President Polk sending American troops into Mexican territory which escalated tensions even more.

The U.S. was hoping for Texas’s annexation, California and New Mexico as a result of a successful war against Mexico. President James Polk sent an American ambassador to negotiate with Mexico but the Mexican government refused his terms, leading to the declaration of war on May 13th, 1846 by President Polk himself.


During this period, many Americans were eager for expansion and believed that diplomatic efforts would be unsuccessful in achieving it; thus they decided upon a military solution instead. The U.S., under President Polk’s orders, had started the conflict with hopes of annexing all or parts of California and New Mexico while also granting Texas its independence from Mexico as part of an overall strategy towards westward expansionism before the Civil War even began.

Cited Sources

- https://www.britannica.com/event/Mexican-American-War 0

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- https://www.history.navy.mil/browse-by-topic/wars-conflicts-and-operations/mexican-american-war.html 2

- http://websites.umich.edu/~ac213/student_projects06/magsylje/history.html 3

- http://peacehistory-usfp.org/us-mexican-war/ 4

- https://digital.lib.niu.edu/illinois/lincoln/topics/mexicanwar 5

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- https://history.state.gov/milestones/1830-1860/texas-annexation 8

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