## Learning Outcomes

- Explain the long-term and immediate causes of the Mexican-American War
- Identify how **Manifest Destiny**, the annexation of Texas, and the border dispute led to war in **1846**
- Describe the conduct of the war across its main military theaters
- Recall key figures, battles, and events including **James K. Polk**, the **Thornton Affair**, and the **Treaty of Guadalupe Hidalgo**
- Analyse why the United States won and why the war became politically controversial
- Connect the war to sectional conflict over slavery in the territories

## AP United States History Syllabus

For the AP United States History, you are expected to understand the causes and conduct of the Mexican-American War, with a focus on the following syllabus points:

- Know that the **Mexican-American War** was fought from **1846 to 1848**
- Identify long-term causes including **Manifest Destiny**, the **1845 annexation of Texas**, and Polk’s desire for **California** and **New Mexico**
- Explain the immediate cause: conflict in the disputed border zone between the **Nueces River** and the **Rio Grande**
- Understand the importance of the failed **John Slidell mission** and the **Thornton Affair**
- Describe the war by major theaters: **Taylor in northern Mexico**, **Kearny and Frémont in New Mexico and California**, and **Winfield Scott’s campaign to Mexico City**
- Know the significance of domestic opposition, including **Lincoln’s Spot Resolutions** and criticism that the war would expand slavery
- Understand the terms of the **Treaty of Guadalupe Hidalgo (1848)** and the creation of the **Mexican Cession**
- Explain how the war intensified sectional conflict through the **Wilmot Proviso** and later debates over slavery in the territories

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why was the border between Texas and Mexico disputed in 1846?
2. How did President **James K. Polk** use the **Thornton Affair** to justify war?
3. Name the three main military theaters of the Mexican-American War.
4. Why did some Whigs and antislavery critics oppose the war?
5. What did the **Treaty of Guadalupe Hidalgo** give to the United States?

## Introduction

The Mexican-American War is one of the clearest APUSH examples of how expansion, politics, and slavery could combine into one crisis. On the surface, it was a war for territory between the United States and Mexico. At a deeper level, it was a test of **Manifest Destiny**, presidential power, and sectional balance. That is why AP questions rarely treat the war as just a list of battles.

For exam purposes, you need to separate three things that students often blur together: the **causes** of the war, the **conduct** of the war, and the **consequences** of the war. The causes explain why fighting began in **1846**. The conduct explains how the United States fought and won. The consequences matter because the land gained from Mexico reopened the issue of slavery’s expansion and helped move the nation toward the Civil War.

> **Key Term: Manifest Destiny**
> The belief that the United States was destined, and often divinely justified, to expand across North America.

### Long-term causes: expansionism, Texas, and Polk’s goals

The war did not begin from one random border clash. It grew out of a chain of expansionist pressures in the 1840s.

The first long-term cause was **Manifest Destiny**. Many Americans believed the republic should spread west to the Pacific. This belief mixed nationalism, racial assumptions, land hunger, and economic ambition. It was not just a slogan. It shaped voting, diplomacy, and public support for expansion.

The second long-term cause was **Texas**. After winning independence from Mexico in **1836**, Texas existed as an independent republic. But annexing it to the United States was politically explosive because Texas would likely strengthen slaveholding power. When the United States finally annexed Texas in **1845**, Mexico did not accept the loss. Mexico still viewed Texas as rebellious territory, not a permanently separate nation.

> **Key Term: annexation of Texas**
> The admission of the Republic of Texas into the United States in 1845, a major cause of rising tensions with Mexico.

The third long-term cause was President **James K. Polk** himself. Polk was an expansionist Democrat elected in **1844**. His administration wanted more than Texas. It wanted **California** and **New Mexico** too. These areas mattered for trade, Pacific access, and national growth. Polk settled the Oregon dispute with Britain diplomatically in **1846**, accepting the **49th parallel**. That matters because it shows choice. Polk was not forced into war everywhere. He chose compromise with Britain and a harder line with Mexico.

Students often miss the difference between a broad cause and a policy aim. **Manifest Destiny** created public support for expansion. Polk translated that support into concrete territorial goals.

> **Test Tip:** In an LEQ or SAQ, build a causal chain rather than listing facts. A strong line is: Manifest Destiny → annexation of Texas → border dispute and failed negotiation → war.

### Immediate causes: the border dispute, Slidell mission, and Thornton Affair

The immediate crisis centered on where Texas ended. The United States claimed the **Rio Grande** as the southern border. Mexico claimed the **Nueces River**. The land between them, sometimes called the **Nueces Strip**, was disputed. That means the key early clash happened on contested ground, not clearly uncontested U.S. soil.

> **Key Term: Rio Grande-Nueces border dispute**
> The disagreement between the United States and Mexico over whether Texas ended at the Rio Grande or the Nueces River.

Polk first tried diplomacy. He sent **John Slidell** to Mexico in **1845** to negotiate the Texas boundary and purchase California and New Mexico. Mexico refused to negotiate. This failure matters because it pushed Polk from purchase to confrontation.

Polk then sent troops under **Zachary Taylor** into the disputed zone. That was a deliberate escalation. If you are writing causation, this is where you should distinguish between **public justification** and **actual causation**. Polk later claimed that Mexico had attacked Americans on American soil. But the location was disputed, which is exactly why critics argued Polk had provoked the fighting.

The immediate flashpoint came in **April 1846** in the **Thornton Affair**, when Mexican troops attacked an American patrol in the disputed area. Polk seized on this event and told Congress that Mexico had “shed American blood upon American soil.” Congress declared war on **May 13, 1846**.

> **Key Term: Thornton Affair**
> The April 1846 clash in disputed territory in which Mexican forces attacked a U.S. patrol, giving Polk the incident he used to ask Congress for war.

This is one of the most exam-tested distinctions in the topic. The Thornton Affair was the trigger, but not the whole cause. A weak answer says “Mexico attacked first.” A strong answer explains that Polk had moved U.S. troops into disputed territory after a failed attempt to buy more land.

Domestic opposition appeared quickly. Many **Whigs** argued Polk had manipulated Congress and the public. **Abraham Lincoln’s Spot Resolutions** demanded to know the exact “spot” where blood had been shed. The point was constitutional and moral: had Polk misled the country into war?

Other critics connected the war to slavery. They feared new territory would expand slave power. **Henry David Thoreau** opposed the war as unjust and linked it to slavery, a protest that later shaped his ideas about civil disobedience.

> **Key Term: Spot Resolutions**
> A set of questions introduced by Abraham Lincoln challenging Polk to identify the exact location where American blood had supposedly been shed on American soil.

### The conduct of the war: three main theaters and key campaigns

A good APUSH answer does not need every battle, but it should organise the war clearly. The best structure is by **theater**.

The first theater was **northern Mexico and Texas**, led by **Zachary Taylor**. Early U.S. victories at **Palo Alto** on **May 8, 1846** and **Resaca de la Palma** on **May 9** helped secure Texas. Taylor later took **Monterrey** in **September 1846**. His most famous victory came at **Buena Vista** on **February 22–23, 1847**, where his smaller force defeated a much larger army led by **Antonio López de Santa Anna**. These victories made Taylor a national hero.

The second theater was **New Mexico and California**. General **Stephen W. Kearny** moved into **Santa Fe**, taking New Mexico with relatively limited resistance. In California, events were more complex than students often think. The **Bear Flag Revolt** in **June 1846** involved American settlers in **Sonoma** declaring a short-lived republic against Mexican rule. But California was not won by settlers alone. **John C. Frémont**, U.S. naval forces, Kearny, and local fighting all played roles.

> **Key Term: Bear Flag Revolt**
> A brief 1846 uprising by American settlers in California against Mexican rule, soon absorbed into the broader U.S. war effort.

The third and decisive theater was **Winfield Scott’s campaign to Mexico City**. This campaign is important because it showed the war had become much more than border defense. Scott launched a major amphibious landing at **Veracruz** in **March 1847**, then marched inland through battles including **Cerro Gordo**, **Contreras**, **Churubusco**, and **Molino del Rey**. The climax came at **Chapultepec** on **September 12–13, 1847**, after which U.S. forces entered **Mexico City**.

This final campaign mattered politically and militarily. Once the U.S. occupied the Mexican capital, it became far harder for Mexico to continue the war. For APUSH, the occupation of Mexico City is the clearest sign that this was a full-scale invasion, not just a frontier skirmish.

### Why the United States won, and why the war mattered politically

The United States won for several connected reasons. It had greater financial resources, stronger naval power, and more effective logistical reach. Its multi-front strategy forced Mexico to defend widely separated regions at once. U.S. commanders also coordinated campaigns more effectively than their opponents.

Mexico, by contrast, faced serious political instability. That did not mean Mexican resistance was weak in every battle. It means the Mexican state struggled to sustain a war against a stronger opponent fighting on several fronts. For exam purposes, avoid saying the war was “easy.” The United States won decisively, but the war still lasted more than a year and a half and required major campaigns.

The war ended with the **Treaty of Guadalupe Hidalgo**, signed on **February 2, 1848**. Mexico recognised the **Rio Grande** as the Texas border and ceded a vast region known as the **Mexican Cession**: present-day **California, Nevada, Utah, most of Arizona and New Mexico, and parts of Colorado and Wyoming**. In return, the United States paid **$15 million** and assumed about **$3.25 million** in claims by American citizens against Mexico.

> **Key Term: Treaty of Guadalupe Hidalgo**
> The 1848 treaty that ended the Mexican-American War, recognised the Rio Grande boundary, and transferred the Mexican Cession to the United States.
>
> **Key Term: Mexican Cession**
> The vast western territory ceded by Mexico to the United States in 1848 after the war.

The war’s biggest APUSH significance lies in what happened next. Even before the war ended, the **Wilmot Proviso** proposed banning slavery in any territory acquired from Mexico. It never passed, but it exposed sharp sectional division. Northern and Southern legislators split mainly by region, not party. That is a major warning sign of the 1850s.

The reason this mattered was simple. Expansion had reopened the old question: would slavery expand westward? The war did not create the slavery debate from nothing. The debate already existed, going back at least to the **Missouri Compromise**. But the scale of land gained in 1848 made the issue more urgent and harder to postpone.

> **Test Tip:** If a prompt asks about the war’s significance, do not stop at “the U.S. gained land.” Push to the next step: Mexican Cession → Wilmot Proviso → sectional conflict over slavery → road to the Civil War.

A subtle but strong evaluative point is this: the war fulfilled much of **Manifest Destiny**, but in doing so it destabilised the party system and intensified sectional politics. That is why the war can be viewed both as a military triumph and as a political disaster.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- The Mexican-American War was fought from **1846 to 1848**
- Long-term causes included **Manifest Destiny**, the **annexation of Texas**, and Polk’s desire for **California** and **New Mexico**
- The immediate crisis came from the **Rio Grande-Nueces border dispute**
- The failed **John Slidell mission** increased tensions by showing diplomacy had broken down
- The **Thornton Affair** gave Polk the event he used to ask Congress for war
- Congress declared war on **May 13, 1846**
- The war was fought in three main theaters: **Taylor in northern Mexico**, **Kearny and Frémont in the Southwest and California**, and **Scott’s campaign to Mexico City**
- Key battles included **Palo Alto**, **Resaca de la Palma**, **Monterrey**, **Buena Vista**, **Veracruz**, and **Chapultepec**
- Domestic critics included **Whigs**, **Abraham Lincoln** with the **Spot Resolutions**, and antislavery opponents such as **Henry David Thoreau**
- The **Treaty of Guadalupe Hidalgo (1848)** ended the war, recognised the **Rio Grande** boundary, and created the **Mexican Cession**
- The war intensified sectional conflict through debate over slavery in the new territories, especially the **Wilmot Proviso**
- For APUSH, the war matters as both an expansionist victory and a major step toward the **Civil War**

## Key Terms and Concepts

- Manifest Destiny
- annexation of Texas
- Rio Grande-Nueces border dispute
- Thornton Affair
- Spot Resolutions
- Bear Flag Revolt
- Treaty of Guadalupe Hidalgo
- Mexican Cession


The Mexican-American War - Causes and conduct of the war