Ancestral Discoveries: Mexican-American War

An often overlooked war in American history is the Mexican-American War. But this historically significant conflict, which ultimately resulted in the territorial expansion of the United States, could provide you with an explanation for where a relative was between 1846 and 1848; why a grandfather or uncle disappeared from the U.S. Census rolls after 1840; or why no burial location can be found for a family member.  

Maybe you’ve already discovered a family member who served in the Mexican-American War. Perhaps you’ve found a pension record and didn’t realize it was connected to this war. Or, you may have seen a grave that displayed the Mexican-American War (aka Mexican War) marker or had its insignia engraved on the monument.   

Family researchers understand that genealogy is so much more than a collection of documents. It’s about a journey of discovery. Part of that experience is an opportunity to place a family member’s life in historical context. This article aims to provide a broad overview of the Mexican-American War and assist researchers learn more if they wish by offering additional resources to explore.   

The Soldiers

An estimated 100,000 American soldiers served during the year-and-a-half-long conflict. United States casualties totaled approximately 17,000 (13,000 dead and 4,000 wounded). Of the deaths, roughly 1,800 died in combat. The others died from sickness and disease. Mexican casualties totaled about 25,000, including as many as 1,000 civilians.

Lithograph by artist James S. Baillie, artillery-fortified U.S. forces under the command of Gen. Zachary Taylor rout Mexican troops during the Mexican American War’s Battle of Buena Vista in February 1847.

Regular U.S. Army regiments included the dragoons, mounted riflemen, artillery, and infantry.  The U.S. Navy squadrons included the Home Squadron and Pacific Squadron. Also involved were the U.S. Marine Corps and U.S. Revenue Marine Service. 

During the war, the Army’s resources became depleted and it was necessary to recruit civilian volunteers to serve short-term enlistments. Volunteer militia regiments were raised in Alabama, Arkansas, California, Florida, Georgia, Illinois, Indiana, Iowa, Kentucky, Louisiana, Maryland and District of Columbia, Massachusetts, Michigan, Mississippi, Missouri, New Jersey, New York, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Texas, and Virginia.

A number of African Americans in the Army during the Mexican–American War were servants of the officers. Also, soldiers from the Louisiana Battalion of Free Men of Color participated in this war. African Americans also served on a number of naval vessels during the Mexican–American War, including the USS Treasure and the USS Columbus.

See a List of U.S. military units that participated in the war. In addition, several state historical and genealogy websites offer detailed rosters of the men who served from their states.

Mexican-American War

The Mexican-American War was a conflict between the United States and Mexico, fought from April 1846 to February 1848. The war followed the annexation of the Republic of Texas by the United States in 1845, which Mexico considered a Mexican territory. Prior to annexation, the Republic of Texas was a de facto independent country.

The 1845 annexation gave rise to a major conflict between the U.S. and Mexico, primarily over where the boundary existed between Texas and Mexico. Mexico asserted the Texas border ended at the more northern Nueces River, while the United States claimed it ended at the Rio Grande.

President Polk’s Plan

President James K. Polk (source Wikimedia Commons)

When President James K. Polk was elected in 1844, he held a strong belief that the United States was destined to expand its borders to the Pacific Ocean, a concept popularized by journalists as America’s “Manifest Destiny.”

His vision included occupation of the Republic of Texas, Oregon, and California.

To achieve his ambitious goal, Polk sought to acquire the lands by peaceful means, if possible; but if necessary, by armed force. 

Faced with the challenge of settling the lingering border dispute between Texas and Mexico, President Polk devised a plan that consisted of three major objectives.

  • First, bring Mexico to the negotiating table willingly and work out a deal where the U.S. would purchase the disputed border land.
  • Secondly, if Mexico refused to negotiate a sale, Polk would position U.S. troops into the border region and try to prevent Mexican goods from crossing the Rio Grande, thereby, cutting off important trade. Polk was convinced that if sufficient pressure was applied, Mexico would join the U.S. at the negotiating table. But he was not a patient negotiator nor interested in a protracted negotiation.
  • And, thirdly, if the first two objectives failed, the U.S. would simply occupy, seize by military force and declare the border area part of the United States.

However, implementing Polk’s third objective would require a Declaration of War. And, Congress was not prepared to issue a declaration without being convinced that it was in the United States’ best interest. Opposition was voiced by key members of Congress who were not in agreement with President Polk’s expansionist vision and his methods to achieve it.

Therefore, to gain Congressional support, Polk would need to strengthen his arguments. So, he devised a provocative scheme consisting of several intentionally antagonistic actions aimed at making Mexico so outraged about the encroachment on their lands that they would become the aggressor. Polk knew that if Mexico attacked first, Congress would grant the declaration of war he needed.  


Fort Brown, established in 1846 as a simple earthwork outpost. Photo courtesy of the Library of Congress.

In a series of antagonistic moves, President Polk ordered the U.S. Army to move its forces south to the Rio Grande. He made demands that the Mexican forces withdraw beyond the Rio Grande. He ordered General Taylor to construct a makeshift fort, later known as Fort Brown/Fort Texas, on the banks of the Rio Grande, opposite the key Mexican city of Matamoros, Tamaulipas.

While Polk was applying his pressure campaign on the ground, he sent two diplomatic teams to try to negotiate the sale of the disputed border land to the United States and prevent a war from erupting.

The first team was part of a covert operation. It was sent to Cuba to try to work out a deal with former Mexican President Santa Anna, who was living in exile there. If he would cut a deal with the U.S. that allowed the sale of the land, they would help him return to a leadership role in Mexico. In addition, he would receive a handsome financial incentive. Santa Anna deceivingly convinced the U.S. officials he wanted to go along with the deal, but would later turn against the United States and lead the Mexican troops in battle.

The other team was sent to negotiate in Mexico with representatives of the current government. Those officials rejected the terms of the negotiation, were not interested in selling the land for the low price the U.S. was offering, and in turn made their own demands that the U.S. “invaders” leave the area and abandon the fort. If the United States refused, there would be consequences.

On April 25, 1846, a 2,000-man Mexican cavalry was dispatched to the contested territory and attacked a 70-person U.S. patrol commanded by Captain Seth Thornton. The U.S. troops were no match for the Mexican forces. Eleven American soldiers were killed and 52 captured in what was to be known as the Thornton Affair.

On May 3, 1846, the Siege of Fort Texas began. The Mexican artillery bombardment on the fort lasted 160 hours before the Mexican troops surrounded it. Thirteen U.S. soldiers were injured and two killed. 

On May 6, 1846, General Zachary Taylor and 2,400 U.S. troops arrived to relieve the fort. What followed was the Battle of Resaca de la Palma on May 9, 1846, in which the two sides engaged in fierce combat. But the U.S. Cavalry managed to capture the Mexican artillery – causing the Mexican side to retreat.

Declaration of War

Once President Polk received word about these battles and the American casualties, he went straight to Congress. In his message on May 11, 1846, Polk stated “Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon American soil.” 

Following a brief period of deliberation, the U.S. Congress approved the Declaration of War on May 13, 1846, with strong support from among southern Democrats. Fourteen Whigs voted no. 

On May 23, 1846, in response to the United States’ declaration of war, Mexican President Paredes issued the Mexican Manifesto of War. 

Multi-Frontal Attacks

The United States military deployed a multi-frontal – surround, infiltrate and conquer – strategy against Mexico.

  • Army troops occupied Santa Fe and claimed the New Mexico Territory for the United States. 
  • A portion of the Army went west to California where they met up with American settlers who had banned together to protect their lands from an ominous threat from the Mexicans. Mexico had issued a proclamation that unnaturalized foreigners were no longer permitted to have land in California and would be expelled. 
  • U.S. Navy Pacific Squadron Commodore John Sloat was ordered to seize San Francisco Bay and blockade California posts. Captain William Mervine landed 350 sailors and Marines at San Pedro. 
  • The First Missouri Mounted Volunteers were sent to modern-day northwest Mexico. Those troops made the precarious 5,500-mile march through an extremely inhospitable area, mostly occupied by Indian tribes, to secure American interests. 
  • Southern Mexico was an area over which the Mexican government had little control. It was mostly populated by indigenous people with closer ties to Cuba and the United States than their own government. The U.S. Navy was able to secure the waters in the Yucatan peninsula area, clearing way for troops and supplies and establishment of another military blockade. 
  • Entering from the south, U.S. forces marched inland into Mexico, where they were victorious in securing several major Mexican cities.
  • Then the troops advanced to Mexico’s capital, Mexico City, which too would fall to American occupation. U.S. General Winfield Scott was named military governor of the territory.  
  • After Mexico City was captured, the Mexican government moved to a temporary capital in Queretaro where numerous hand-to-hand battles occurred.

A Peace Process

Now, with a weakened military and many of its cities occupied, including its capital, the Mexican government had no option other than to bring an end to the war. Mexico made a series of negotiation attempts, but since the United States held the upper hand, it would be the U.S. to decide on what peace would entail.   

Weighing on the minds of U.S. Congressional members was the fact that the war had cost many American lives. Those sacrifices were made while heroically battling to occupy key Mexican cities and areas of that county. Strategically, they understood the U.S. now controlled the Yucatan Peninsula and had it own governor in charge in Mexico City.

Therefore, the most significant question before Congress was whether the U.S. should annex the entirety of Mexico. But, after much heated debate and fierce objections, Congress decided to reject that idea.

The 1848 Treaty of Guadalupe Hidalgo was signed Feb. 2, 1848. It was ratified, with amendments, by the U.S. Senate on March 10, 1848.

In addition to ending the war, the treaty: 

  • Gave the U.S. undisputed control of Texas.
  • Established the U.S.-Mexican border along the Rio Grande.
  • Ceded to the United States present-day, California, Nevada, and Utah, most of New Mexico, Arizona, and Colorado, and parts of Oklahoma, Kansas, and Wyoming. 

In return, Mexico received: 

  • A $15 million payment for physical damage to Mexico.
  • Forgiveness of $3.25 million in debt owed by the Mexican government to U.S. citizens.
  • In addition, Mexicans living in the ceded areas had the right to become U.S. citizens. And any land they held under Mexican law, would be considered legitimate land grants in the United States.  

Further Learning

Article sources


The Dead March: A History of the Mexican-American War, by Peter Guardino
The Mexican War, 1846-1848, by K. Jack Bauer and Robert W. Johannsen
A County of Vast Designs: James K. Polk, the Mexican War and the Conquest of the American Continent, by Robert W. Merry

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