How the Mexican-American War Began

Texas-Mexico border 1846

Map courtesy of Wikicommons

History has many examples of one nation trying to impose its will on other countries. One example is the Mexican-American War where many historians often ignore or distort details of what led the US Congress to declare war against Mexico on May 13, 1846.

After his election in 1844, on a platform that included Texas statehood, President Polk was determined to acquire the ports of San Francisco and San Diego along with vast portions of Mexico from the Pacific Ocean to Texas. However, Mexico repeatedly refused his offer of $25 million to buy California.

Official maps at that time showed the Texas border between the US and Mexico was the Nueces River that emptied into the Gulf of Mexico at Corpus Christi. In January of 1846, President Polk ordered US troops at Corpus Christi to move more than 100 miles south to the Rio Grande river where they began building fortifications that became Fort Brown near what is now Brownsville. Polk and his Secretary of War William Marcy believed that Mexico would consider the troop movement and fortifications an invasion of its territory and would feel pressured to comply with the expansionist desires of the United States to avoid further military action.

Polk had drafted a declaration of war and he called his cabinet members together on Saturday, May 8, to consult with them. At the end of the meeting, he decided to send his war message to the Congress on Tuesday, May 11. But later that same evening, he received word that 52 US troops had engaged a Mexican cavalry unit after entering a Mexican ranch on the Rio Grande on April 25. Several Americans were killed and a few were wounded in the short skirmish that lasted until early the next day, and which became known as The Thornton Affair for the name of the commanding officer.

The president quickly revised his war message to include his view about the significance of the battle, and sent the message to Congress on Monday, May 10. It asserted that Mexico “…has invaded our territory and shed blood of Americans upon the American soil.” The House expedited a war resolution and approved it on May 13 with only 14 dissenting votes, and the Senate concurred in a 40-2 vote. The US and Mexico were officially at war.

Declaring war against Mexico divided the country, as evidenced in leading newspapers of the time. Walt Whitman editorialized in support of the war, and volunteers responded to advertisements and posters stating the US government was offering recruits generous pay and 160 acres of land.

Mexican War(1)

Some prominent political leaders, including John Quincy Adams, opposed the war. Anti-war organizations denounced the war, particularly after news of the seven-day US naval bombardment at Veracruz that killed hundreds of civilians. After that incident, Henry David Thoreau refused to pay taxes to support the war, went to jail, and later published his famous essay “On Civil Disobedience.”

As a first-term congressman opposed to the war, Abraham Lincoln researched and presented his famous “Spot Resolutions” in Congress in 1847 and risked his political career by accusing Polk of lying to Congress about the basis for declaring war. Several times he challenged Polk to show him the spot where American blood was shed, implying that it was on Mexican soil and that the US soldiers were invaders. However, by then, Mexico City had already fallen to US troops and all that remained was a formal surrender and signing the Treaty of Guadalupe in February 1848 that officially established a new border stretching from San Diego to the mouth of the Rio Grande.

Ulysses S. Grant, an Army captain in the invasion and subsequent occupation of Mexico, used his memoirs to call the Mexican-American War “the most unjust war ever waged against a weaker nation by a stronger.”

Today’s textbooks use terms such as “Westward Expansion” and “Manifest Destiny” to obscure how and why the US used its military superiority to acquire nearly half of Mexico as a result of the war. The conquered Mexican territory included what is now California, Nevada, Utah, and Arizona, along with parts of New Mexico, Colorado, Wyoming, and Kansas. The US paid Mexico $18.5 million as reparations, less than what it offered for California before the war.

Most historians also gloss over Polk’s actions and how he misled the Congress. The truth is in the Congressional Record and in battlefield journals, some of them stored in archives in both the USA and Mexico. Only a few historians have tried to research the documents and present the facts.

The book Abraham Lincoln and Mexico by Michael Hogan, an historian/ educator in Guadalajara, Mexico, presents these facts and includes many of these archival documents in their entirety so educators and the public can understand the factual history of how the Mexican-American War began. It’s a great way to learn from the past and stimulate discussion of ways to move forward in relations between the USA and Mexico.

The print version of Dr. Hogan’s 2016 book is in the Abraham Lincoln Presidential Library and in university and public libraries across the USA, and even in foreign countries. In the past two years, the Lincoln and Mexico Project (LAMP) has sent free print copies of the book to all members of the US Senate and distributed free copies of the eBook version to more than 400 educators in the USA, Mexico, and other countries as supplemental classroom material. Educators can request the free eBook by writing to

Author: LAMP

Co-founder of the Lincoln and Mexico Project. Email: USA: 619-246-4342. MX: +52-33-3676-5897

2 thoughts on “How the Mexican-American War Began”

  1. I am ashamed as an American after reading this. I’ve lived 65 years and I can still be shocked by what our country has perpetrated on others. Who do we think we are? We are no better than those who have marched into countries globally and forcefully taken them. This mentality has got to end! I don’t know what else to say but stop this ignorance NOW!


    1. TY for the feedback, Nancy Meyers. These facts are missing from many textbooks, and that’s why we are providing the eBook version free to educators as supplemental classroom material. Best regards.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.