Mexican-American War: Definition, Causes, Significance, & Outcome

Facts about the Mexican-American War

The Mexican-American War was a bloody war fought between the U.S. and Mexico from 1846 to 1848. The armed struggle holds a very important place in the history of the United States, as it was the first major war the U.S. fought on a foreign land.

Essentially the war saw a very aggressive and territory-hungry United States face off against an ill-prepared Mexican force. The commander in chief of the U.S. at the time was President James K. Polk, a Manifest Destiny enthusiast who was determined to realize America’s westward expansion to the Pacific Ocean at all cost.

Leading the U.S. on the field of battle was the astute general Zachary Taylor (later 12th president of the U.S.). At the end of the war, the U.S.’s superior armed forces claimed large Mexican territories. The territory annexed was estimated at more than 520,000 square miles (1,300,000 square km), spanning all the way from the Southwest to Pacific coast lines.

Here is a quick look at the causes, significance, and outcome of the Mexican-American War:

What was the Mexican-American War, and how long did it last?

The Mexican-American War began as a border skirmish between the United States and Mexico that spiraled into a full-blown out war between April 1846 and February 1848.

The war was clearly fueled by the expansionist policies – which were underpinned by the Manifest Destiny – of then-U.S. President James K. Polk. Having secured an emphatic win over Mexico, the U.S. helped itself to additional 500,000 square miles of territories. In exchange of the territory, which went all the way from the Rio Grande to the Pacific Ocean, Mexico was compensated with a meagre $15 million.

The Mexican-American War was a classic case of a superior nation bullying a weaker nation into utter submission. This point is reflected in U.S. general and later 17th President of the United States U.S. Grant’s assertion that the war was “one of the most unjust ever waged by a stronger against a weaker nation”.

Causes of the Mexican-American War

The primary cause of the Mexican-American War was America’s annexation of the Republic of Texas in 1845. Following Texas annexation, the two countries entered into a bitter dispute over where Texas border ended. Mexico, which by then had cut off diplomatic ties with the U.S., claimed that the Texas border ended at the Nueces River; while White House claimed that the border was instead at the Rio Grande.

Prior to the annexation, Texas had fought very hard to gain its independence (during the Texas Revolution 1835-1836) from Mexico.

After several attempts to resolve the border dispute between Texas and Mexico collapsed, the U.S. – under the presidency of James K. Polk – took a bold move and seized by force the territories around the Rio Grande.

What drove the U.S. into annexing the Republic of Texas?

Annexation of Texas happened under a James K. Polk administration that wholeheartedly held the view that it was the U.S.’s destiny to expand its borders all the way to the Pacific Ocean. This view was known back then as the Manifest Destiny.

The concept of Manifest Destiny was coined in 1845 by John L. O’Sullivan, the editor of the United States Magazine.

Opponents of the Mexican-American War and the Wilmot Proviso

The war between the U.S. and Mexico in the mid-1840s was primarily opposed by the Whig Party. The Whigs described the war as an affront to everything that the Founding Fathers of the United States stood for, calling the war a reckless grab of foreign lands.

The Whig Party, which at the time controlled the House, voted in favor of a resolution that tagged President James K. Polk’s actions as unconstitutional and unnecessary.

Furthermore, many people in the largely anti-slavery North, as well as abolitionists, including Frederick Douglass, opposed President Polk’s war against Mexico, calling it as an attempt by the president to extend slavery into the soon-to-be-annexed territories.

Dreading the possibility of this happening, David Wilmot – Representative from Pennsylvania – introduced an amendment to an appropriations bill on August 8, 1846. Known as the Wilmot Proviso, the amendment sought to prohibit slavery in all the territories seized from Mexico. Unfortunately, the Wilmot Proviso failed to get passed in Congress. Regardless, Whig members of Congress continued to voice out their opposition throughout the war.

In response to the criticisms from the Whigs, the Democratic president stated that he was left with no other option than to attack Mexico after Mexican troops had attacked U.S. troops on American soil. Polk even went to the extent of calling opposition Whig Party politicians of engaging in treason.

In December 1847, Abraham Lincoln, who was then a House Representative from Illinois, worked very hard investigating where exactly the attack on U.S. troops had taken place.

Lincoln, who later served as the 16th president of the U.S. and the hero of the American Civil War, wanted to find out whether the territory where the attack occurred was indeed part of the United States. His investigations were known back then as the “Spot Resolutions”. Lincoln’s resolutions failed to get traction in the House, allowing President Polk to keep his war efforts going.

Henry David Thoreau authored a very brilliant essay titled “Civil Disobedience” (1849), arguing that the U.S. war against Mexico was immoral.

Did you know: Critics of President Polk’s war against Mexico called his war efforts a despicable land grab, terming it as “Mr. Polk’s War?

Famous African American soldiers during the Civil War

Mexican forces attack U.S. troops

Following the U.S. annexation of Texas in 1845, Mexico responded by cutting off ties with the U.S. in March that year. What the annexation meant was that the U.S. was now neck deep in Texas’s border dispute with Mexico.

President James K. Polk dispatched U.S. diplomat John Slidell to Mexico to resolve the border dispute. Slidell was also tasked to make an offer for the purchase of New Mexico and California for about $30 million. Then-President of Mexico José Joaquín Herrara was livid by the actions of the U.S., and he refused to welcome Slidell.

After news of Mexico’s snub reached the White House, President Polk sent about 4,000 American troops, under the command of General Zachary Taylor, to the disputed territory. The territory, which Mexico claimed was theirs, was between the Nueces River and the Rio Grande.

Seeing the presence of American troops on their soil as blatantly aggressive, Mexican troops attacked U.S. troops at the Rio Grande on April 25, 1846. The attack left about a dozen U.S. troops dead, while 16 were injured.

Mexico received a huge boost after their forces was taken over by the decorated Mexican general Antonio López de Santa Anna. | Image: Santa Anna in a Mexican military uniform

The U.S. declares war on Mexico

Mexican-American War

Less than three weeks after the attack, President Polk sent a message to Congress in May, 1846. The president’s message called on Congress to swiftly come to action, stating that Mexico had “shed American blood on American soil”.

After the war broke out, Mexico received a huge boost after their forces was taken over by the decorated Mexican general Antonio López de Santa Anna. The Mexican general had reneged on his promise to Polk to push for peace, only for him to take command of the Mexican forces.

The United States army was divided into two commands – one under Gen. Zachary Taylor and the other under Col. Stephen Kearny. Taylor was ordered to make his way into Mexico and try to reach Mexico City; while Kearny was ordered to attack New Mexico and California.

Kearny had very little difficulty taking New Mexico and California; however, Gen. Zachary Taylor had to dig in and fight bravely in many battles before capturing Monterrey. In February 1847, General Taylor secured an important victory at the Battle of Buena Vista.

Battle of Buena Vista

The fall of Mexico City

Displeased by Gen. Taylor’s lack of ruthlessness and urgency to march into the heart of Mexico, President Polk sent Gen. Winfield Scott to capture Veracruz and march straight to Mexico City. Gen. Winfield Scott carried out Polk’s instructions, taking Veracruz in March 1847 before capturing Mexico City on September 14, 1847. Comprehensively beaten by a superior armed force, the Mexican forces put up little to no resistance to Scott’s army.

Treaty of Guadalupe Hidalgo and Outcome of the Mexican-American War

Following the U.S. triumphant victory over Mexico in the Mexican-American War, the Treaty of Guadalupe Hidalgo was signed between the two countries on February 2, 1848.

Under this peace treaty, the U.S. secured more than 1,300,000 square km (520,000 square miles) of territory that originally belonged to Mexico. The additional territory increased the size of the United States by about one-third.

The ceded territories to the U.S. include present-day U.S. states of Utah, Nevada, Arizona, Texas, California, and parts of Colorado. The price for those territories came at $15 million to the American tax payers. Mexico did however have to pay some compensation to Americans for the damage inflicted during the war.

Another very important outcome of the Mexican-American War was that it added more fuel to the fire of sectionalism in the United States. Since the 1820s, Americans had been divided over expansion of slavery to newly admitted states into the union. This disagreement often pitted the largely anti-slavery politicians in the North against pro-slavery ones in the South.

The Mexican-American War boosted the political career of Zachary Taylor, who was considered by many as a national hero. Buoyed on by his national fame, Taylor went on to contest in the 1848 presidential election, emerging victorious. The former general was sworn in as the 12th President of the United States in April 1849.


It’s been stated more U.S. troops died from diseases and infection than on the battle field. About 10,000 U.S. soldiers died of diseases and infections; while about 2,000 soldiers actually perished on the battle field.

In addition to losing close to half of their original territory, Mexico lost about 25,000 people in total, including civilians.

The Mexican-American War gave many of the generals (both Confederate and Union generals) that would fight in the American Civil War ample experience. Example of Confederate generals that fought in the Mexican-American War included the likes of Robert E. Lee, George Pickett, James Longstreet and Thomas “Stonewall” Jackson. On the other hand, Union generals such as Ulysses S. Grant, George Gordon Meade and Joseph Hooker fought valiantly in the Mexican-American War.

General Ulysses S. Grant, who went on to become the 18th President of the United States, had damning comments to say about the Mexican-American War. The general described it as “one of the most unjust” wars in U.S. history.

Did you knowRobert E. Lee, a leading general in the Confederate army, served as a captain in the Army Corps of Engineers during the Mexican-American War? General Lee distinguished himself brilliantly in the Battles of Cerro Gordo and Contreras.

Other Interesting Facts about the Mexican-American War

The Battle of Monterrey, September, 1846

Mexican-American War | The Battle of Monterrey September 20–24, 1846 |Painting by Carl Nebel

  • Majority of President Polk’s support and the support for the Mexican-American War came from within his Democratic Party, particularly Democrats in the Southwest.
  • General Winfield Scott’s invasion of Veracruz from the sea was the largest amphibious operation conducted by the U.S. until World War II. That particular operation saw the United States ferry more than 10,000 U.S. troops in under five hours.
  • The six Mexican military cadets that refused surrendering and who went on to fight to their bitter deaths at Chapultepec Castle hold a special place in the history of Mexico. Those teenaged cadets were called “Niños Heroes” or “Hero children”.
  • Less than a week before the Treaty of Guadalupe Hidalgo, which saw Mexico cede California to the U.S., gold was discovered in the newly acquired territory.
  • Poor sanitation in the army camps were responsible for the spread of infections and diseases that ultimately killed more American soldiers than the war actually did. Diseases like yellow fever, smallpox, mumps, and measles were the commonest.
  • Plans for full annexation Mexico couldn’t come to fruition because of Nicholas Trist negotiated a lesser settlement for the United States in the Treaty of Guadalupe Hidalgo. Trist was the chief negotiator of the Treaty of Guadalupe Hidalgo. He defied Polk’s order to return home and signed the treaty with the Mexicans.
  • Before the Treaty of Guadalupe Hidalgo was signed, the U.S. had to wait for the formation of a new Mexican government.
  • Following U.S. victory, the Rio Grande became the U.S.-Mexican border and not the Nueces River.
  • Other names of the Mexican-American War include: Guerra de 1847, Guerra de Estados Unidos a Mexico (“War of the United States Against Mexico”), and Mexican War. The war has also been termed as America’s “forgotten War”.

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