The Causes and Reasons for the Mexican-American War of 1846-1848

The Mexican-American war of 1846-1848 was not inevitable but both sides placed themselves on a collision course that seemingly made it so.   A combination of Mexican unwillingness to recognize Texas independence and the desire of Texans for statehood with American desire for westward expansion set the stage for the first offensive war in the short History of the United States.   Tensions between Mexico and the United States had been building for decades, ever since the Mexican government invited Anglo settlers into Texas in the 1820’s.   The war with Mexico was the result of long-standing Anglo grievances that were mostly of the Mexican government’s own making.   Perhaps the single greatest factor that led to war aside from American land-hunger was the instability of the Mexican government in the decades prior to the war.

The Anglo settlers of Texas were initially invited to settle Texas by the Spanish government of Mexico in 1821.   The first colony of 300 families was led into Texas by Stephen F. Austin and they settled to a depth of 150 miles along 100 miles of coastline near present day Houston.[1] The colony flourished despite the revolution in Mexico of 1821 in which the Spanish were ejected and a short-lived Mexican Empire was established.   A federal constitution was ratified in 1824 in which the Texans were guaranteed to keep their land take part in the governance of Mexico.

The first major grievance between the Texans and the Mexican government came in 1831 when the dictatorial regime of Bustamente threatened to deprive the Anglo settlers of their holdings.   He also repealed the colonization law and stopped the flow of settlers into Texas, sending troops into Texas to terrorize the people already settled there.   With the elevation of Santa Ana to the Mexican presidency in 1832, the treatment of the Anglos in Texas worsened.   This all culminated in the Texas war of independence in 1836.   Texas formally declared independence on 2 March 1836, and after initial reverses at the Alamo and Goliad, they defeated Santa Ana at the Battle of San Jacinto on 21 April 1836.   After San Jacinto, Santa Ana signed the Treaty of Velasco on 14 May 1836, which provided for the removal of Mexican troops from Texas territory but it did not specifically recognize Texas’ independence.

The settlers in Texas were not the only people with grievances against the Mexican government.   The United States government had a list of claims they had been pressing against the Mexican government since the early 1830’s.   There were numerous instances of the Mexican authorities at various ports and the Mexican government was seemingly powerless to stop the abuses.   Twice joint commissions between U.S. and Mexico met to determine the legitimacy and method of repayment for claims and both times the effort met with failure.   Mexico refused to pay the claims or made a small effort to pay the claims and then defaulted on the agreed amount.[2] Despite all of this the American government continued to attempt to negotiate with the Mexican government to prevent further abuses of American vessels and citizens while trying to get payment for the claims of American citizens against the government of Mexico.

When the United States offered a treaty of annexation to the people of Texas in 1844, the Mexican government cut off diplomatic contact with America, even though the treaty was subsequently rejected by the U.S. Senate.   This was nothing new, throughout the previous two decades the Mexican government had been consistent in it offensive behavior towards American envoys no matter which party was in power in Mexico at the time.   America had continually acted in good faith with the different Mexican regimes in order to secure just compensation for the claims of American citizens against the Mexican government.

That the United States wished to expand to the Pacific Ocean was no secret.   The term “Manifest Destiny” was coined by a New York newspaper reporter, and it perfectly described how most Americans felt about the rights of their nation to acquire more territory.[3] Land hunger was not new to America and would not end with the Mexican-American War.   Prior to the war America had acquired the Louisiana Territory by purchase from France and had driven the Spaniards from Florida and simultaneously with war were contesting the possession of Oregon on the Pacific Northwest with England.   There was a very real danger of the U.S. having to fight two wars at once.

The biggest problem with the Mexican government in the years leading up to the war was its very instability, which kept Mexico from maintaining a consistent foreign policy.   Between 1821 and 1846, the Mexican government changed hands no less than eleven times.[4] These changes were usually the result of a palace revolution and the common people had little if anything to do with the running of their own country.   The immediate cause of Mexican resentment was discontent with their loss at the hands of the Texan revolutionaries in 1836, with which they could never reconcile themselves.   The Mexicans maintained a simmering border war with Texas from 1836 until the outbreak of the war, the source of which was a border dispute.   Texas claimed the Rio Grande River as its southern boundary while Mexico claimed the Nueces River that was almost 100 miles further north was the boundary.   In 1843, the Mexican Foreign Minister plainly warned the American envoy that any attempt to annex Texas would mean war.   This was one position that every Mexican government was forced to take, for a government that ceded Texas would lose legitimacy at home and would surely be quickly overthrown.

The United States offer of annexation to Texas was done quickly in the final days of the Tyler administration in December 1846 by a joint resolution of Congress.[5] James Polk, the new president, had been an ardent proponent of annexation in his election campaign and was determined to see the annexation through to its completion.   The quickness with which this was done enraged the Mexican government and seemingly left them no choice but to follow through on their threats of war.

The causes of the Mexican American War are many and complex but one thing is plain.   America and Texas were determined to join, and Mexico was equally determined to prevent that from happening.   It is possible that Mexico could have been brought to recognize Texas’ independence and eventually even their annexation by the United States.   What brought war on was the quickness and rapidity of American action combined with the Mexican efforts to forestall the annexation of Texas by the United States.   A combination of Mexican unwillingness to recognize Texas independence and the desire of Texans for statehood with American desire for westward expansion set the stage for the first offensive war in the short History of the United States.


The Avalon Project at Yale Law School, “Treaty of Velasco, 14 May 1846”,

The Avalon Project at Yale Law School, “Joint Resolution of the Congress of the United States, December 29, 1846”,

Brooks, N.C., A Complete History of the Mexican War: Its Causes Conduct, and Consequences: Comprising an Account of the Various Military and Naval Operations.   From its Commencement to the Treaty of Peace, First published, 1849, Cranbury, NJ: The Scholars Bookshelf, 2006

Singletary, Otis A., The Mexican War, Chicago: The University of Chicago Press, 1960


[1] Brooks, N.C. A Complete History Mexican War: Its Causes, Conduct, and Consequences: Comprising and Account of the Various Military and Naval Operations from its Commencement to the Treaty of Peace. First published in 1849, reprinted by: Cranbury, NJ: The Scholar’s Bookshelf, 2006, pp. 24-25

[2] Ibid. pp. 21-23

[3] Singletary, Otis. The Mexican War, p. 14

[4] Ibid. pp. 17-18

[5] The Avalon Project at Yale Law School, “Joint Resolution of the Congress of the United States, December 29, 1846”,