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Mexican-American War

Causes of the War

American Homefront During the War
Effects of the War
Causes of the War



       The Mexican-American War had several causes, some of which major and others of which minor.  The most important, however, were the Texas crisis, westward American movement, the ideal of Manifest Destiny, and the actions carried out by the President Tyler and President Polk. 

      The first cause of the Mexican-American War was Texas and California.  When Texas broke away from Mexico in 1836, Mexico did not recognize it as an independent republic but as a corrupted territory.   This territory, they believed, was nonetheless still a part of Mexico.  The government of the United States believed differently, however.  As his time in office was coming to an end, President John Tyler received enough support to annex Texas.  Mexico was enraged as Texas entered the Union as the 28th state and believed that the annexation of Texas was a call for the necessity of war (“Mexican-American War”). 

Furthermore, when President James Polk had John Slidell travel to Mexico offering a sum of 30 million dollars for New Mexico and California, Mexico grew angrier for they would gain nothing from the loss of Mexico if they were to complete the sale.  Mexican officials never met with Slidell, however, for they refused to accept him (“Mexican-American War,” 1).  Then on January 13, 1846, General Zachary Taylor and his troops were ordered to travel down to the Rio Grande to maintain the disputed border of Texas (Mexicans believed that the Texas border was the Nueces River while Texans and the U.S. believed it to be the Rio Grande)(“Countdown to the War,” 1).   Mexico, thinking that Taylor and his men were on its land, attacked the American unit in an act of self defense (Heys, 1).  Soon after these events, the United States declared war on Mexico on May 13, 1846 (“Mexican-American War,” 1).  

Another cause of the Mexican-American War was the westward movement of citizens of the United States.  At this date and time Americans were constantly looking for land to start farms or sell to incoming farmers.  They found the most easily accessible and available land in Texas.  There was also the ideal that the Southerners traveled into Texas in order to get another slave state into the Union and relinquish Southern power (Heys, 1).  Nonetheless, as waves of people from the U.S. poured into Texas, they began to undermine the authority of the Texans and the Mexican government.  As if this were not enough, the invaders disobeyed Mexican laws and regulations such as the antislavery laws.  These behaviors combined greatly contributed to the disproval of U.S. citizens by Mexican officials/law makers (“Mexican-American War,” 1). 

Next, there was the ideal of Manifest Destiny, which is the American ideal that the United States should stretch from ocean to ocean as one massive nation.  This ideal greatly contributed to the westward migration of Americans as they traveled across the continent in search of economic gain (Heys, 1).  The greatest example of expansion for economic gain was seen in the United States’ desire to acquire California in an effort to establish markets and an economic base in heavily populated Asia (“Mexican-American War”). 

     Lastly, there were the motions of the presidents who were in office at the time when Mexican and American tensions were rising.  First, there was President Tyler.  In an effort to gain support from voters for the upcoming election, Tyler supported the annexation of Texas.  In fact, Tyler was the one that got the joint resolution to annex Texas passed, which was the main reason for Mexican hostility.  Then, there was President Polk.  After running on a platform centered on expansionism, Polk put his ideals to work encouraging western expansion and the ideal of Manifest Destiny.  After Mexico refused to negotiate Slidell and attacked U.S. troops along the Rio Grande, Polk asked Congress to declare war on Mexico, once and for all starting the war (Heys, 1).   

     In conclusion the causes of the Mexican American War were actually quite clear and simple.  First, there was the Texas crisis and the argument over land.  Then, there was the never halting westward advance of citizens onto Mexican territory.  Finally, there was the ideal of Manifest Destiny, which almost all U.S. citizens believed in at the time.  When all of these occurrences were combined, enough had been done to spark a war; a war of which was important to U.S. land expansion. 





Works Cited

“Countdown to War.”  A Coneise History of the U.S.-Mexican War.  August 8, 2004.  Internet.  Online.  Available <>  

Heys, John.  “Enough Blame to Go Around: Causes of the Mexican-American War.” 1995.  Internet.  Online. Available <>

“Mexican-American War.”  May 30, 2006.  Internet.  Online. Available <>






Mexican-American War Home


A picture of the Alamo (a landmark of the Texas Revolution)


President Tyler


President Polk


General Taylor