Lesson 3: War With Mexico

Lesson Focus

President James K. Polk's drive for territorial expansion led to war with Mexico. This lesson explores how his determination to annex California, despite political opposition, fulfilled America's expansionist goals and reshaped the continent.

People to Know

James K. Polk

Learning Objectives

• Compare the United States' immediate and long-term objectives for engaging in the war with Mexico.
• Identify the key terms and territorial changes established by the Treaty of Guadalupe Hidalgo, which officially ended the war.

President James K. Polk aimed to annex California, fearing Britain might seize the valuable region. In late 1845, he sent diplomat John Slidell to Mexico to offer up to $25 million for the territory.

However, Mexican officials, outraged by the recent US annexation of Texas, refused to negotiate with Slidell.

This complete diplomatic failure and Mexico’s breaking of relations made a peaceful acquisition impossible, pushing both nations toward armed conflict.

The immediate cause of war was a dispute over the Texas border. The US claimed the Rio Grande, while Mexico insisted on the Nueces River.

President Polk ordered General Zachary Taylor into this disputed zone. In April 1846, Mexican troops crossed the Rio Grande and attacked a US cavalry unit.

Polk used this clash to argue that Mexico had started the war, leading Congress to declare war on May 13, 1846.

President Polk planned a three-part military strategy to ensure victory.

First, General Zachary Taylor invaded northern Mexico. Second, General Stephen Kearny captured New Mexico and helped secure California. The final phase began when Mexico still refused to surrender.

General Winfield Scott marched his forces from Veracruz and successfully captured Mexico City in September 1847, which effectively ended the major fighting of the war.

The Treaty of Guadalupe Hidalgo, signed in February 1848, officially ended the war.

Under its terms, Mexico ceded a vast territory that included California and New Mexico, and it recognized the Rio Grande as the southern border of Texas.

In return, the United States paid Mexico $15 million and agreed to pay over$3 million in debts owed to American citizens. Note that Mexico lost half its national territory.

Even after the war, one final piece of land was needed to complete the nation's continental expansion.

In 1853, the United States negotiated the Gadsden Purchase with Mexico for $10 million.

This acquisition of a strip of land in modern-day Arizona and New Mexico was important because it provided an ideal southern route for a transcontinental railroad, finalizing the borders of the continental United States.