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The Mexican-American War: A Fulfillment of Manifest Destiny

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Battle of Churubusco fought near Mexico City on the 20th of August 1847. One of the final battles of the Mexican-American War.

Battle of Churubusco fought near Mexico City on the 20th of August 1847. One of the final battles of the Mexican-American War.

Manifest Destiny: North American Expansion

Though it was a small war by most standards and has been largely forgotten by the public, the war between Mexico and the United States in the mid-1840s greatly affected both nations. Americans were pushing westward, seeking more land to build their dreams of freedom tethered from their past.

The editor of The United States Magazine and Democratic Review gave the movement its name in 1845, when he wrote that it was “the fulfillment of our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions.”

Manifest Destiny was America’s duty to subdue North America, spread the ideals of liberty and freedom, all with the blessing of the Almighty. There were only two large problems standing in the way of Americans overrunning the continent: namely, Mexico and Great Britain.

The northwest part of the continent, called the Oregon Country, was held by Great Britain, who would eventually relinquish much of their land to the United States after careful negotiations and a treaty. The territories that now make up Texas, California, and all points between were held by Mexico.

When the U.S. offered to purchase the land, Mexico refused, not willing to give up their territory. America would eventually gain this territory that stretched to the Pacific coast, but it would be at the cost of many thousands of young men’s lives on both sides of the border.

Map of the United States showing the westward expansion from 1815 to 1845.

Map of the United States showing the westward expansion from 1815 to 1845.

Causes of the Mexican-American War

Texas, formerly a northern province of Mexico, had broken away from Mexico in 1836 and formed the Republic of Texas, which was recognized as an independent nation by the United States, Great Britain, France, and other countries. Texas had petitioned the United States to join the Union as a state, prompting Mexico to threaten war if the annexation of Texas occurred.

James K. Polk became president of the United States running on a platform as an expansionist, which included adding Texas as a new state. Shortly after Polk’s inauguration, in March 1845, Mexico—in protest of the annexation of Texas—withdrew its minister and severed diplomatic ties with the U.S.

The Mexican government, though they were hardly prepared for war with the United States, took a belligerent approach partly because it felt it had a strong hand. The Mexicans believed the U.S. was going to be drawn into a war with Great Britain over the hotly contested Oregon Territory.

If war between the United States and Great Britain did break out, Mexico was planning on becoming an ally of Great Britain, putting the United States in a weak position for negotiating acquisition of territory.

The war with Great Britain was avoided through peaceful negotiations over the Oregon Territory, thus undermining Mexico’s position. Mexico found itself in the position of having to choose between selling California to the United States or engaging in a war to sustain its pride and territorial integrity.

President Polk wanted to bring the land west of Missouri into the Union through peaceful means if possible; if not, then it must be war.

Map of the Republic of Texas circa 1842.

Map of the Republic of Texas circa 1842.

Flash Point of the War

In contention between the United States and Mexico was the exact border between the state of Texas and Mexico. Texas claimed her western boundary was the Rio Grande River to its source and the northern to the 43 degrees north latitude. Mexico claimed the actual boundary between the two was the Nueces River, some one hundred miles eastward.

Texas was fully admitted into the Union by a joint resolution of Congress in December 1845, provided that the boundary dispute was settled with Mexico. President Polk sent the 61-year-old General Zachary Taylor to the disputed area with over three thousand troops.

Polk also authorized Taylor to call on the governor of Texas to reinforce him with such militia as “may be needed to repel invasion or to secure the country against apprehended invasion.”

The Mexican forces clashed with Taylor’s forces in the disputed region on April 25, 1846; 11 U.S. soldiers were killed, five wounded, and 47 were captured. Taylor sent an immediate dispatch to Washington stating, “Hostilities may now be considered as commenced.” Polk sent a message to Congress asserting the war had begun since “American blood has been shed on American soil.”

After a contentious debate in Congress, war was declared with Mexico. Some northern Whigs condemned the declaration of war, asserting that the war was simply a way to acquire more slave territory and denied that the disputed area belonged to the United States.

Other factors contributed to America’s willingness to go to war. For many years Mexico had been in a chronic state of revolution; as a result, American citizens in Mexico had sustained property losses and were frequently unjustly arrested and harassed by Mexican authorities. Claims against the Mexican government had been settled in part.

President Polk dispatched John Slidell as the United States minister to Mexico to settle the boundary dispute and the unpaid claims by American citizens. The Mexicans publicly claimed they were ready to settle both disputes diplomatically but refused to meet with Slidell once he arrived in Mexico City.

Polk was offended that the president of Mexico refused to receive his minister, which justified in Polk’s mind the possession of the disputed territory by General Taylor and his soldiers. Polk met with his cabinet and formulated a strategy to invade New Mexico, capture Santa Fe, then conquer California.

In addition, General Taylor would drive the Mexican forces south of the Rio Grande River and out of the disputed territory. Polk assumed once U.S. troops were in place in California, New Mexico, and the southern border that Mexico would have no other choice but to concede to the American demands.

The Mexicans were more resolute in defending and retaining their territory than Polk gave them credit for. Mexico had won its freedom from Spain less than three decades before and was in no position for war, possessing only a small naval coast guard and 30,000 poorly trained troops in their army. The U.S. was not ready for war either, with only 8,500 soldiers in their army.

Sheer numbers did not tell the whole story as the Mexican army was poorly trained and equipped. Many of their commanders held honorific commissions but knew little about the art of war. On the other hand, the U.S. army had competent officers and more modern equipment, were well trained, and had a uniform supply system.

Unlike the Mexican army, many of the U.S. military officers had received formal training in military matters at the U.S. Military Academy at West Point, New York. Though much smaller in numbers, the U.S. military was superior to the Mexican army.

President Polk called for a buildup of the army through recruitment of thousands of volunteers; an initial wave of war fever swept the nation. Dozens of state volunteer regiments were formed, leading to a fighting force capable of covering such a wide expanse of territory. Before the war’s end, over 73,000 volunteers would serve in the military.

General Zachary Taylor rides his horse at the Battle of Palo Alto - May 8, 1846.

General Zachary Taylor rides his horse at the Battle of Palo Alto - May 8, 1846.

Battle at Palo Alto

The first battle of the war was fought above the Rio Grande at Palo Alto, near modern day Brownsville, Texas. Leading the troops was veteran commander Zachary Taylor, who had been a professional solider since 1808.

Taylor’s forces clashed with 6,000 troops of the Mexican Army of the North led by General Mariano Arista on May 8, 1846. The intense battle lasted four hours, with Arista forced to retreat.

The next day, Taylor’s men found the Mexicans in a defensive position along an old path of the Rio Grande, the Resaca de la Palma. Taylor’s assault broke the Mexican lines, causing panic that Arista and his officers could not contain. Taylor’s victory resulted in over 600 Mexican casualties, with his forces suffering about a third as many deaths.

During their hasty retreat further south into Mexico, Arista’s troops dropped their arms and supplies along the way. With his initial success, Taylor moved his army deeper into Mexico occupying Matamoros, Mexico, on May 17, and then pushing onto Camargo.

Taylor’s men would be victorious at battles in Monterrey and Saltillo in the fall of the year. The war with Mexico was the first U.S. war fought on foreign soil that was widely covered in the press. Taylor’s exploits won him national fame as a military leader and would eventually pave his way into the White House.

Colonel Kearny Captures New Mexico

Occurring at the same time as Taylor was forcing his way deeper into Mexico, U.S. forces invaded New Mexico and California. Under President Polk’s order, Colonel Stephen Kearny led the campaign against Santa Fe, New Mexico, with troops he had marched from Fort Leavenworth, in the Kansas Territory.

Kearny’s total force was 1,600 men, a mix of regular army troops and volunteers. Kearny and his troops arrived in Santa Fe in mid-August and found the city virtually defenseless. Over the next few weeks an additional 1,000 volunteers joined Kearny to begin the overland march from Santa Fe to California.

The Conquest of California

In the 1840s, hundreds of Americans settled in the Sacramento Valley in California. Though the area was part of Mexico it was considered a remote province and had little Mexican government oversight.

The U.S. Army mapmaker, John C. Frémont, entered California on an exploratory mission with a contingent of sixty well-armed men. The Mexican authorities feared Frémont and his men and ordered them to leave. Frémont fortified a hilltop east of Monterey and raised the American flag. To avoid warfare with the Mexicans, he then fled north to Oregon.

The Mexican provincial government issued a proclamation ordering all foreigners out of California, which included the hundreds of American settlers who had already put down roots in the area.

The settlers turned to Frémont with their concerns, but he failed to act. The frustrated settlers took the initiative and seized a herd of horses going south for use by the Mexican army. Next, they captured Sonoma in June of 1846, which was an important Mexican stronghold north of the San Francisco Bay.

The main Mexican official in Sonoma, General Mariano Guadalupe Vallejo, having received little or no help from the government in Mexico City, joined the Americans. The rebels proclaimed California as an independent state and raised their banner, the Bear Flag.

In July, Frémont stepped in and took control of the situation, lowered the Bear Flag, replacing it with the Stars and Strips. By the end of 1846 nearly all of California was under American control and Frémont was regarded as a hero who had won the “Golden Gate” by his efforts.

In early December, Colonel Kearny arrived near Los Angeles, which was still under Mexican control. On December 5, at San Paucal, Kearny with sailors and Marines from Commodore Robert Stockton and Frémont’s men defeated a Mexican detachment of 600 at San Gabriel and captured Los Angles.

A third offensive was taking place against El Paso del Norte (modern day Juarez, Mexico) led by a colonel of the Missouri volunteers, Alexander Doniphan. The Missourians defeated a Mexican force twice their size north of El Paso on Christmas Day 1846.

While occupying El Paso, Doniphan waited for reinforcing artillery and then marched toward Chihuahua, taking the city by defeating a much larger Mexican contingency.

Mexican General Antonio Lopez de Santa Anna.

Mexican General Antonio Lopez de Santa Anna.

A New Phase of the War

Though the war had gone decidedly in favor of the Americans, the Mexican government was refusing to admit defeat. Without a treaty that confirmed Mexico’s loss of California and New Mexico, the United States could not officially lay claim to the territory as it was still in dispute.

President Polk conferred with the general in chief of the U.S. Army, Winfield Scott, to develop a plan to end the war and gain control of the new territory. The plan was to capture the capitol city of Mexico, Mexico City. Polk ordered Scott to assemble a strong expeditionary force by taking many of Taylor’s regulars and adding several thousand volunteers and a few hundred U.S. Marines.

General Taylor’s temper flared when he realized he was being eased out of the prominence of the war, and was stuck maintaining control of Northern Mexico with a much smaller force while the new offensive was being planned to capture the capitol.

Scott, as well as being a practiced military officer with 40 years of service, was a military scholar who had studied the great wars of Europe in detail, as well as written many of the standard training manuals for the U.S. military. Scott immediately set about making plans to capture Mexico City.

He had special wooden landing boats built to carry the soldiers from the ships offshore to the beach at the Mexican port city of Veracruz. The first step of the campaign would be to capture the city of Veracruz and set up a U.S. base of operation. Scott’s force landed at Veracruz in early March of 1847.

As the U.S. prepared for the amphibious landing at Veracruz, the Mexicans were busy building their army. The new president of Mexico, General Antonio Lopez de Santa Anna, set about the ambitious plan of creating an army of 25,000 soldiers.

The years of war and internal struggle within Mexico had depleted their treasury, which left Santa Anna with little funds to equip and train his new army. A letter was captured from General Scott that detailed his plans in Mexico, giving Santa Anna vital information.

Santa Anna intended to defeat Taylor’s much smaller army of 5,000 encamped at Buena Vista ranch, near Saltillo, and then return to Mexico City to defend the city from Scott’s troops.

The Battle at Buena Vista and the March to Mexico City

Santa Anna marched his army 400 miles over rough terrain in winter to reach the encamped Americans at Buena Vista. One February 22, 1847, Santa Anna’s army attacked Taylor’s army in a series of piecemeal assaults, which failed to rout the Americans.

The Mexican forces launched an attack on the American lines but were repulsed by the Mississippi volunteers led by Colonel Jefferson Davis. Santa Anna’s army did not give up easily; however, after continued rebuffs by the Americans they began a hasty retreat to Mexico City.

The battle at Buena Vista had been a crushing defeat for Santa Anna. Forty percent of his army was either dead, injured, or missing. Taylor’s troops suffered much less, losing only 700 men.

Once in Mexico City, Santa Anna appealed to the Mexican people to rally his army and began to conscript soldiers using new taxes and money taken from the Catholic Church. Scott’s army reached Veracruz on March 9 and laid siege to the city, capturing the city within three weeks.

The Americans set up their base at the port city and by early April Scott and his army began the march to Mexico City along the National Road.

Scott first encountered Santa Anna’s troops about 50 miles from Veracruz at Cerro Gordo. Santa Anna deployed 11,000 of his troops at a natural defense point in the city. Rather than falling prey to Santa Anna’s strengths, Scott deployed his troops in a flanking maneuver under the able command of his junior officers, Robert E. Lee, P.G.T. Beauregard, and George B. McClellan.

Scott’s approach was successful and by mid-April Santa Anna was in retreat. The U.S. forces lost 425 during the encounter; Mexican losses were 1,000 killed or wounded and 3,000 taken as prisoners.

Though Scott’s army was victorious in battle, they were facing many internal problems that weakened the army. The warmer climate in southern Mexico was a natural breeding ground for diseases and 1,000 American troops lay ill in a hospital in Veracruz with an additional thousand ill at Jalapa, a few miles west of Cerro Gordo.

In addition to the ravages of disease, Scott was losing his troops to the end of their enlistments. Most of his army were volunteers who had enlistment periods of a few months, and thousands of the enlistments expired in June. Once the volunteers’ service was completed, they returned to their farms and family. Scott had no choice but to halt his army at Puebla while he waited for reinforcements.

Scott’s small army of 7,000 men forced him into a decision that could prove to be disastrous; he did not have enough troops to man supply garrisons along the National Road to Veracruz. The American troops now had to retreat or press on without a supply line and live off the land.

Scott chose the latter; however, he had learned many important lessons during his extensive study of the European wars. He established good relations with the local mayors and the clergy of the Catholic Church, thus ensuring the necessary food and materials needed for his army. Scott’s policy of appeasement of the local population also resulted in few guerrilla style attacks on his camps.

U.S. Army occupation of Mexico City in 1847. The American flag is flying over the National Palace.

U.S. Army occupation of Mexico City in 1847. The American flag is flying over the National Palace.

The Battle for Mexico City

With a force of 10,000 soldiers, Scott marched his men to the outskirts of Mexico City, arriving in mid-August 1847. Santa Anna mustered a force of 25,000 troops, mostly new untrained recruits, and positioned them throughout the city.

Once again, Scott, rather than advancing on Santa Anna’s stronger positions, moved up from the south over the terrain that the Mexican general considered impassable, thus giving the Americans the advantage by attacking areas of Santa Anna’s lines that were lightly manned.

The offensive consisted over a series of attacks and counter attacks that lasted over a month. Though ultimately successful, Scott’s army suffered heavy casualties, with nearly a third of his army killed, wounded, or suffering from disease.

On September 14 the victorious U.S. forces entered the plaza of downtown Mexico City, which ended the bloody campaign. American forces occupied and subdued the city over the next several months.

Treaty of Guadalupe Hidalgo

After victories in Mexico City, northern Mexico, and California, the Mexican government had no alternative but to concede defeat. Negotiations began with the ambassador, Nicholas Trist, sent by President Polk and Mexican officials. It would take months of discussion before a treaty could be reached.

In February 1848 in Guadalupe Hidalgo, a village near Mexico City, a treaty was finally reached. The Treaty of Guadalupe Hidalgo was very favorable for the Americans, giving them vast tracts of land in the western parts of North America. The land that was annexed became known as the Mexican Cession.

Over time the states of California, New Mexico, Arizona, Nevada, Utah, and parts of Colorado and Wyoming would be brought into the Union. Mexico lost about half of the land area but only a small portion of its population. The Texas-Mexico border dispute was settled with the Rio Grande River forming the boundary between Texas and Mexico.

For all this land, the United States agreed to pay Mexico $15 million and assume all claims of American citizens against the Mexican government, over $3 million. The war was not without other costs to the Americans as over 10,000 soldiers had died from combat or disease and $100 million had been expended to fund the conflict.

San Francisco harbor circa 1850. The congestion in the harbor often forced ships to wait days before unloading their passengers and cargo.

San Francisco harbor circa 1850. The congestion in the harbor often forced ships to wait days before unloading their passengers and cargo.

Consequences of the War

The war with Mexico greatly expanded the territory of the United States, which now stretched from the Atlantic Ocean to the Pacific Ocean. Just months after the end of the war gold was discovered in California, prompting hundreds of thousands to flock into the region in search of their fortunes. The mass migration to California hastened the process of becoming a state, which was granted in 1850.

The many officers who were graduates from the U.S. Military Academy in West Point, New York, served with distinction and helped solidify the role of the academy in the military. The Marines who served in the war received praise for their valor, which helped give credence to their role in warfare and secured for that branch of the military continued funding from Congress.

Many political careers were launched as a result of service in the war. President Polk, who had been very involved in directing the war, broadened the power of the presidency as commander in chief of the military. General Zachary Taylor became a hero of the war, which propelled him into the White House in the election of 1848.

The Whig party later nominated General Scott as their nominee in the 1852 presidential election, but he lost to a former subordinate Franklin Pierce. The Democrat Pierce, a New Hampshire politician, had served in the war, rising to the rank of brigadier general. The young and handsome Pierce easily won the election over the aging general Scott.

The vast new territory acquired by the United States added fuel to the ongoing debate over slavery. The thorny issue, which had been debated since the early days of the republic, sometimes with fiery rhetoric, remained unresolved.

To temper the bitter animosity between those opposed to slavery in the North and slavery supporters in the South, Congress passed a series of acts that became known as the Compromise of 1850.

As a result of the legislation, California was admitted as a free state but allowed slave owners to bring slaves into the Western territories that were captured from Mexico in the war. In addition, the Compromise ended the slave trade in Washington, D.C., and provided a new Fugitive Slave Law.

The Mexican-American war was controversial at the time and remained so for years to come. Ulysses S. Grant, who served in the war and later became president of the United States, called the war “one of the most unjust ever waged by a stronger nation against a weaker nation.”

Even though America benefited greatly from the outcome of the war, the cost was high in blood and treasure. The ideal of Manifest Destiny had been fulfilled as nearly 300,000 Americans would take the arduous journey to settle the West Coast by the start of the Civil War.


  • Chambers, John Whiteclay II. The Oxford Companion to American Military History. Oxford University Press. 1999.
  • Eisenhower, John S.D. So Far from God: The U.S. War with Mexico 1846-1848. University of Oklahoma Press. 2000.
  • Henderson, Timothy J. A Glorious Defeat: Mexico and its War with the United States. Hill and Wang. 2007.
  • Tindall, George Brown and David Emory Shi. America: A Narrative History. Seventh Edition. W.W. Norton & Company. 2007.
  • West, Doug. The Mexican-American War: A Short History, America’s Fulfillment of Manifest Destiny. C&D Publications. 2020.
  • West, Doug. James K. Polk: A Short Biography: Eleventh President of the United States. C&D Publications. 2019.

© 2020 Doug West


Doug West (author) from Missouri on January 21, 2020:


Thanks for the comment. For a war most people have never heard of, it really changed the United States.

Tim Truzy from U.S.A. on January 21, 2020:

Splendid and well written, Doug. I truly liked the way you told the story of this war. This was a pleasant read.