A simple guide to a big fight that changed the map.

Imagine you wake up and your map looks different. New lines. New names. Long ago, that really happened. Two neighbors argued about a fence. The fence was a border. The argument became a war.

Short answer: The Mexican–American War was a war between the United States and Mexico from 1846 to 1848. They fought over land in the West and where Texas’s border should be. It ended with the Treaty of Guadalupe Hidalgo, and the U.S. gained a lot of land in the Southwest.

The Quick Story You Can Tell a Friend

Texas joined the United States. Mexico did not agree on the border. The U.S. said the border was the Rio Grande. Mexico said it was the Nueces River. Soldiers met in the space between. Shots were fired. War began.

Why Did It Start?

• Texas: Texas had fought for independence from Mexico in the 1830s. In 1845, it joined the U.S.
• The Border: Two rivers, two answers. Rio Grande or Nueces? This big question started the fight.
• More Land: U.S. leaders wanted land to the Pacific Ocean. Many people called this “expansion.”

Who Was Involved?

• United States: President James K. Polk sent troops. Generals like Zachary Taylor and Winfield Scott led battles.
• Mexico: Mexican soldiers fought to protect their land. Leaders, including Antonio López de Santa Anna, took part.

What Happened During the War?

Fighting took place in today’s Texas, New Mexico, California, and deep into Mexico. The U.S. won key battles in northern Mexico and captured Mexico City in 1847. After that, both sides agreed to talk peace.

How Did It End?

The war ended in 1848 with the Treaty of Guadalupe Hidalgo. Mexico agreed to the Rio Grande border for Texas. Mexico also gave up a large area of land to the U.S. The U.S. paid Mexico money for this land.

What Land Are We Talking About?

• Today’s California, Nevada, Utah
• Most of Arizona and New Mexico
• Parts of Colorado and Wyoming

(A few years later, in 1853, the U.S. bought a small extra slice called the Gadsden Purchase.)

Why Does It Still Matter?

• Maps Changed: The shape of the U.S. and Mexico we see today grew from this war.
• People and Culture: Spanish, Indigenous, and Mexican roots stayed strong in the Southwest. We see it in place names, food, art, and language.
• Big Questions: The war raised hard debates about power, borders, and fairness—topics we still talk about.

Fast Facts

• Dates: 1846–1848
• Main Cause: Border dispute and U.S. expansion
• Treaty: Guadalupe Hidalgo (1848)
• New Border for Texas: Rio Grande
• Result: U.S. gained the Southwest; Mexico kept its independence

Think About It Like a Fence

Two neighbors can share a street and be kind. But if they argue about the fence line, things get tense. Good maps, clear rules, and fair talks help people live side by side. That lesson still counts today.

Simple Q&A

Was it only about Texas? No. Texas started the fight, but the war became about much more land in the West.

Did the war change both countries? Yes. The U.S. became larger. Mexico faced a great loss but kept its nation and culture, which still shine today.

The Takeaway in One Line

The Mexican–American War was a short, important war that drew today’s border lines and shaped life, culture, and maps across North America.

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