A short, clear guide to a big, important war—told simply.

Imagine two neighbors who cannot agree where the fence should go. They both say, “This land is mine!” Feel the tension? That is how the Mexican–American War began. It was a real war between two countries, not just a fence fight.

Answer in one line: The Mexican–American War was a conflict between Mexico and the United States from 1846 to 1848, mainly over where the border was and after the U.S. added Texas; it ended with the Treaty of Guadalupe Hidalgo, which set the Rio Grande as the border and gave the U.S. large western lands.

What Started The Fight?

Texas is the spark. The United States accepted Texas in 1845. Mexico did not agree. Mexico said the border was the Nueces River. The U.S. said the border was the Rio Grande. Two lines. One land. Big problem. When soldiers met in the space between the rivers, shots were fired. War began.

Where Did It Happen?

Fighting happened in many places. Battles were in Texas and northern Mexico. Ships and soldiers went to the east coast of Mexico, at Veracruz. Troops marched to Mexico City. There were also fights in New Mexico and California. It was wide and far.

Who Led Each Side?

In the U.S., President James K. Polk wanted more land in the West. Two main generals led soldiers: Zachary Taylor in the north and Winfield Scott from the coast to Mexico City. In Mexico, General Antonio López de Santa Anna returned to lead. Many other brave people fought on both sides.

What Were The Big Battles?

Early on, the U.S. won at Palo Alto and Resaca de la Palma near the border. In the north, there were battles at Monterrey and Buena Vista. On the coast, the U.S. took Veracruz. Then they moved inland. The fight at Chapultepec opened the way to Mexico City. Each battle pushed the war toward an end.

How Did The War End?

The war ended with a peace deal called the Treaty of Guadalupe Hidalgo in 1848. Mexico and the U.S. agreed the border was the Rio Grande. The U.S. paid money to Mexico and also took over some money claims. The fighting stopped. The map changed.

What Changed After The War?

The U.S. gained a huge area called the Mexican Cession. This included today’s California, Nevada, Utah, most of Arizona and New Mexico, and parts of Colorado and Wyoming. Mexico kept its heartland but lost much land. The U.S. grew fast. Gold was found in California soon after, and many people rushed west. The map you see in school today comes from this treaty.

Why Did This War Matter?

It mattered for both countries. For the U.S., new land raised hard questions: Would slavery spread there? People argued, and those arguments helped lead to the U.S. Civil War later. For Mexico, the loss was painful and changed politics and pride for years. For people living in the new lands, life changed overnight. New laws. New rulers. Old homes stayed, but flags changed.

Voices You Might Not Hear In Textbooks

Tejanos, Californios, and many Indigenous nations lived in these places before the war. The treaty said they could keep property and become U.S. citizens. In real life, many still lost land and rights. History is not only maps and dates; it is families and farms and futures.

Simple Timeline You Can Trust

• 1845: The U.S. accepts Texas. The border is disputed.
• 1846: Fighting begins near the Rio Grande. Congress declares war.
• 1847: U.S. forces take Veracruz and then Mexico City.
• 1848: Treaty of Guadalupe Hidalgo ends the war. The new border is the Rio Grande. The U.S. gains western lands.

Key Ideas In Plain Words

• Border Dispute: Mexico said the border was the Nueces River; the U.S. said it was the Rio Grande.
• Annexation of Texas: The U.S. brought Texas in; Mexico disagreed.
• Treaty of Guadalupe Hidalgo: Ended the war in 1848 and redrew the map.
• Mexican Cession: The big land area Mexico gave to the U.S. after the war.

How To Explain It To A Friend

Two countries argued about a line on a map after Texas joined the U.S. Soldiers fought from the border to Mexico City. In the end, they signed a treaty. The line moved to the Rio Grande. The U.S. got huge western lands. Mexico lost land but kept its core. Many people’s lives changed in big ways.

A Friendly Way To Remember

Think “T-B-T”: Texas, Border, Treaty. First Texas joins the U.S. Then they fight over the border. Then comes the treaty that sets the Rio Grande and changes the map. Texas, Border, Treaty—easy as 1-2-3.

What We Can Learn Today

Borders are not just lines. They are homes, farms, rivers, and stories. Clear talk and fair deals can stop fights before they start. Listening to many voices helps us see the whole picture. History is a teacher when we keep our minds open.

One-Sentence Wrap-Up

The Mexican–American War was a two-year fight, started by a border dispute after Texas joined the U.S., and it ended with a treaty that set the Rio Grande as the border and gave the United States large western lands—changing both countries forever.

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