Mexican American War  1846 - 1848

The Situation after the Independence of Texas  

    The Border Question    

Not so secret Negotiations

 Mexican President Overthrown  

 Polk orders Zachary Taylor to the border  

Taylor on the Rio Grande

 The Thornton Affair  

 War is Declared


Prelude to the Mexican American War



The Situation after the Independence of Texas 


The United States recognized the independence of Texas in 1837 . Many congressmen believed that annexing Texas too soon would led to war with Mexico . The American president after Texas won its independence. Andrew Jackson , was not for immediate annexation . " Prudence," said he, " seems to dictate that we should still stand aloof, and maintain our present attitude, if not till Mexico or one of the great foreign powers shall recognize the independence of the new Government, at least until the lapse of time, or the course of events, shall have proved, beyond all dispute, the ability of that country to maintain their separate sovereignty, and to uphold the Government constituted by them."



So Far from God: The U.S. War With Mexico, 1846-1848

This well-written, comprehensive history of the war takes into account the political and diplomatic dimensions as well as the military.


President Tyler


The following president, Van Buren. also felt America was not ready to go to war with Mexico over Texas .The matter of Texas admission to the United States also became embroiled in the slavery issue. If Texas was admitted to the Union it would become a slave state and northern states opposed its annexation and  the Whig party in the north were nearly united in their opposition to the annexation of Texas. It required a majority of two-thirds of the Senate to annex a foreign territory in accordance with the provisions of the Constitution ; and that, as half of the Senators represented free States, such a majority was at present unattainable and was proved when the measure failed in June 1844 .It was discovered by President Tyler,who was pro-annexation, that what could not be effected by treaty, could as well be performed by a joint resolution of the two houses of Congress. Such a resolution required only a bare majority in each branch .No time was lost by Tyler in making the choice offered to him by the joint resolutions. On the 3rd March, a few hours before his term of office expired, he dispatched a messenger to the American agent in Texas to propose the resolution of annexation to the acceptance of the Texan Government .On the 4th July, Texas consented to be annexed, and the December 29, 1845 , it joined the Union .


A Glorious Defeat: Mexico and

Its War with the United States

A survey of the Mexican War from a Mexican perspective


American manifest destiny


But more support for annexing Texas and other territories was growing. There was a fear England would purchase California, an under this mistaken belief that Britain actually had purchased California, the American Commodore Thomas Catesby Jones seized Monterey, California in 1842 for a day before returning it took Mexican rule .


President James Polk


In 1844 James Polk won the presidency on a platform that included annexation .Prior to Polk's election, president John Tyler introduced an annexation resolution to Congress, which passed the House of Representatives in January 1845 and the Senate in the next month . Texas joined the Union on December 29, 1845 .


As soon as the joint resolution annexing Texas passed the Unites States Congress, the Mexican minister to the U.S. lodged a formal protest and asked for his passport . The Mexican Senate broke relations with the United States on March 28, 1845 and gave Herrera authority to raise troops and prepare for war . The new president, President Polk ordered army troops to the border and sent navy ships to the Mexican coast . Not wishing to be labeled a war eagle, he made one last effort at peace by sending John Slidell to try to negotiate with Mexican president Jose Joaquin Herrera .


The Border Question 



The matter of prime importance was the boundary dispute between western Texas and Mexico .Throughout Spanish and Mexican history, the western border of the Texas territory had been the Nueces River . In 1836, the Congress of the Republic of Texas claimed the Rio Grande as the western boundary . They based this on the facts that when Texas was under Mexican rule, the Mexican government had allowed some Americans settle in the territory between the Nueces and the Rio Grande and that when Santa Anna agreed to withdraw his troops after his defeat and capture at San Juncinto, he withdrew them across the Rio Grande, as per the Treaties of Velasco . However, the Mexican government never formally accepted the treaty . The Americans also claimed the right of self defense against Indian raids from Indians that were said to inhabit the disputed area .The Texans claimed the Rio Grande to its source, which included parts of  modern day New Mexico, Oklahoma, Kansas, Wyoming, and Colorado.


President Jose Joaquan de Herrera


Not so secret Negotiations 


Slidell also carried secret instructions to try to purchase California ( Mexican Alta California ) and the remainder of New Mexico (Nuevo Mexico). $5,000,000 was offered for the New Mexico territory and $25,000,000 or more for California . The Mexican press soon found out these secret details and when it became common knowledge in Mexico it caused an uproar . Rebellion was threatened if President Herrera negotiated with the Americans to sell Mexican soil .

Military opponents of President Jose Joaquan de Herrera, supported by populist newspapers, considered Slidell's presence in Mexico City an insult. After a more nationalistic government under General Mariano Paredes y Arrillaga came to power, the new government publicly reaffirmed Mexico's claim to Texas; Slidell, convinced that Mexico should be "chastised," returned to the United States.

Mexican President Overthrown 


Mariano Paredes y Arrillaga

While the U.S. was plotting to invade, the Mexicans could not unite in the face this danger and continued their old patterns of conservative-liberal strife. Herrera, with much difficulty, was able to assemble a force of 6,000 men. This was put under the command of the ultra-conservative General Mariano Paredes y Arrillaga and sent to the north to San Luis Potosi to reinforce Mexican troops along the border . Paredes got as far as San Luis Potosi, but instead of marching north against the invaders, in December 14, 1845 he rose in revolt of President Herrera. Paredes  was infuriated that Herrera would even allow the American envoy into Mexico .Paredes entered Mexico City on January 2, 1846. On the following day he was named president of Mexico by a junta of notables he had assembled from heads of governmental departments.

Polk orders Zachary Taylor to the border 

Zachary Taylor's army in Corpus Christi

President Polk still needed a stirring casus bell for popular support for the war and wanted a show of force to help in the negotiations of Slidell to buy California instead of having to resort to arms . He ordered General Zachary Taylor into the disputed area on August 30 with the seventh regiment of infantry and three companies of dragoons ( Dragoon is the traditional name for a soldier trained to fight on foot but who transports himself on horseback ) and militia from Alabama, Mississippi,Louisiana, Tennessee, and Kentucky,about 4,000 in all He was told, " The assembling of a large Mexican army on the borders of Texas, and crossing the Rio Grande with a considerable force, will be regarded by the Executive as an invasion of the United States and the commencement of hostilities.In case of war, either declared or made manifest by hostile acts, your main object will be the protection of Texas ; but the pursuit of this object will not necessarily confine your action within the territory of Texas. Mexico having thus commenced hostilities, you may in your discretion cross the Rio Grande, disperse or capture the forces assembled to invade Texas, defeat the junction of troops uniting for that purpose, drive them from their positions on either side of the river, and, if deemed practicable and expedient, take and hold possession of Metamoras and other places in the country."

General Zachary Taylor

General Taylor, instead of proceeding immediately to the Rio Grande agreeably to his instructions, stopped at Corpus Christi at the mouth of the Nueces, the extreme point of Texas proper, and Oct. 4th, 1845, wrote to the Secretary, "Mexico having as yet made no positive declaration of war, or committed any overt act of hostilities, I do not feel at liberty under my instructions, particularly those of July 8th, to make a forward movement to the Rio Grande without authority from the war department." As there was no invasion to repel, and as his march into the Mexican territory in time of peace would be an act of aggression, he prudently waited for further orders.

On the 12th January, 1846, the first dispatch was received from Slidell in Mexico, from which it appeared probable that, although the Mexican Government had not yet refused to receive him, it would enter into no negotiation with him, except in reference to Texas. It had been hoped that Mexico would agree to sell California in exchange for the claims against Mexico .The very next day peremptory orders were sent to Taylor to advance to the Rio Grande to try to provoke the Mexican forces in Laredo or Matamoros

General Taylor, in pursuance of orders, commenced his march into the Mexican territory. Not an American, not a Texan was to be found South of Corpus Christi. After proceeding through the desert about one hundred miles, he met " small armed parties of Mexicans who seemed disposed to avoid us." On approaching Point Isabel, a Mexican settlement, and the site of a Mexican Custom House, he found the buildings in flames. At the same time he received a protest from the " Prefect of the Northern District of Tamaulipas'' against his invasion of a territory " which had never belonged to the Texas, an invasion of which no notice had been given to the Government of Mexico, and for which no reason had been assigned. The protest concluded with assuring Taylor that, so long as his army " shall remain in the territory of Tamaulipas, the inhabitants must, whatever professions of peace 'you may employ, regard you as openly committing hostilities, and for the melancholy consequences of these they who have been the invaders must be answerable in the view of the whole world."


Rare Photographs From The Mexican-American War

Taylor on the Rio Grande 

On the 28th March, Taylor, without having met with the slightest opposition, planted his standard on the bank of the Rio Grande and placed a battery of eighteen pounders one the east bank of the Rio Grande, opposite Matamoros and started constructing a fort known as Fort Texas, later known as Fort Brown .

The new Mexican president Mariano Paredes y Arrillaga was not intimidated by this show of force. He expelled the U.S. envoy from Mexico, declared his willingness to fight, and sent thousands of troops to the city of Matamoros. In April 1846, he appointed General Mariano Arista as commander of the Army of the North to fight the Americans.

Five days after our arms had thus threatened and insulted Metamoras, General Ampudia reached the city with reinforcements, and immediately addressed a letter to the American General, complaining that his advance to the Rio Grande had ' not only insulted but exasperated the Mexican nation," and requiring him within twenty- four hours to remove his camp, and retire beyond the Nueces ; adding, '" If you insist on remaining upon the soil of the department of Tamaulipas, it will clearly result that arms, and arms alone, must decide the question." As Taylor had been sent to Tamaulipas expressly to produce this very result, he took occasion of this letter to hasten the desired crisis.He therefore resorted to an expedient which would compel Ampudia to fire the first shot, and thus, according to the wishes of the Cabinet, to make the intended war, one of defence, "a war by the act of Mexico." There were two American armed vessels at Brazos Santiago, and these he ordered to blockade the mouth of the Rio Grande, thus cutting off all communication with Metamoras by sea. " It will at any rate compel the Mexicans to withdraw their army from Metamoras where it cannot be sustained, or to assume the offensive on this side of the river."

Notwithstanding the blockade, the Mexicans did not attack Taylor ; whereupon he determined, it seems, not to remain any longer idle. Accordingly, the very day on which' he informs the Secretary that the relations between himself and the Mexicans remained the same, and when not a single shot had been fired by the latter, he reports, " with a view to check the depredations of small parties of the enemy on this side of the river, Lieutenants Dobbins of the 3d Infantry, and Porter, 4th Infantry, were authorized by me a few days since to scour the country for some miles with a select party of men, and capture and destroy any such parties that they might meet. It appears they separated, and that Lieutenant Porter at the head of his own detachment surprised a Mexican camp, drove away the men, and took possession of their horses." In this affair, Pofter and one man was killed whether any, or how many Mexican lives were sacrificed, does not appear .

The Thornton Affair 

His next letter of 26th April, reports, on April 24 , what was to be known as the Thornton Affair occurred which gave Polk his cause for war . Taylor wrote "that a party of dragoons sent out by me on the 24th instant to watch the course of the river above on this bank, , ' engaged with a very large force of the enemy, and, after a short affair in which some sixteen were killed and wounded, appear to have been surrounded and compelled to surrender."


It appears that Captain Seth Thornton the commander of the party of with 70 dragoons , acting on the advice of a local guide , investigated an abandoned hacienda, discovered a small body of Mexicans on the summit of a rising ground, about 25 miles from the U.S. camp . He immediately charged upon them ;" but the main body of about 2,000 Mexican soldiers under the command of Colonel Anastasio Torrejon were on the other side of the hill, and therefore unseen, coming up captured the assailants after a battle of a few hours. Another letter, published in the Philadelphia Inquirer , says, " 16 Americans, including Captain Seth Thornton were killed and an unknown number of Mexicans were killed .49 Americans were taken prisoner and held at Matamoros, Tamaulipas.

War is Declared 

General Taylor, after mentioning the affair in the words we have given, announces to the Cabinet the attainment of the long desired result. " Hostilities may now be CONSIDERED AS COMMENCED." Upon the Strength of this despatch, the President announced to Congress and the world, " Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil. On May 13, 1846, Congress declared war on Mexico, despite protests by the Mexican government . Mexico officially declared war on July 7 .



 President James Polk: Mexican–American War Speech

 Listen to and read President James Polk’s U.S. - Mexican War message to the U.S. Congress on May 11, 1846. In this speech, Mr. Polk explained why the United States should declare war upon Mexico.



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