Learn on PengiPengi Social Studies (Grade 8)Chapter 4: The Age of Jackson and Westward Expansion (1828–1850)

Lesson 3: Manifest Destiny and the Mexican-American War

In this Grade 8 Social Studies lesson from Pengi's chapter on the Age of Jackson and Westward Expansion, students define Manifest Destiny by analyzing John Gast's painting "American Progress" as a primary source. They then examine the causes of the Mexican-American War and evaluate the territorial gains the United States acquired through the Treaty of Guadalupe Hidalgo.

Section 1

Analyzing "American Progress"

The concept of Manifest Destiny—the belief that the United States was divinely ordained to expand from the Atlantic to the Pacific—is famously captured in John Gast's painting, American Progress. The artwork features a giant angelic figure named Columbia floating westward, stringing telegraph wire and carrying a schoolbook.

The visual symbolism is clear: the East is painted in light, representing "civilization," technology (trains), and progress. The West is shrouded in darkness, where Native Americans and buffalo are shown fleeing before the advance of white settlers. This image justified expansion as a righteous mission to bring Technology and enlightenment to the "wild" continent.

Section 2

Conflict on the Rio Grande

After the U.S. annexed Texas, tensions with Mexico exploded over the border. Mexico claimed the border was the Nueces River, while the U.S. claimed it was the Rio Grande, much further south. This created a large disputed zone between the two rivers.

President James K. Polk sent General Zachary Taylor’s troops into this disputed territory. When Mexican soldiers fired on them, Polk claimed Mexico had "shed American blood upon American soil." Congress declared war in 1846. Critics, including Abraham Lincoln, questioned the war, but the desire for land drove the conflict forward.

Section 3

The Mexican Cession

The war ended with the U.S. victory and the signing of the Treaty of Guadalupe Hidalgo in 1848. Under this treaty, Mexico was forced to cede (give up) 55% of its territory to the United States.

This vast land acquisition is known as the Mexican Cession. It included present-day California, Nevada, Utah, Arizona, and parts of New Mexico and Colorado. While the U.S. achieved its dream of stretching to the Pacific Ocean, the acquisition of this new land immediately reignited the explosive debate over whether slavery would be allowed in the West.

Lesson overview

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Section 1

Analyzing "American Progress"

The concept of Manifest Destiny—the belief that the United States was divinely ordained to expand from the Atlantic to the Pacific—is famously captured in John Gast's painting, American Progress. The artwork features a giant angelic figure named Columbia floating westward, stringing telegraph wire and carrying a schoolbook.

The visual symbolism is clear: the East is painted in light, representing "civilization," technology (trains), and progress. The West is shrouded in darkness, where Native Americans and buffalo are shown fleeing before the advance of white settlers. This image justified expansion as a righteous mission to bring Technology and enlightenment to the "wild" continent.

Section 2

Conflict on the Rio Grande

After the U.S. annexed Texas, tensions with Mexico exploded over the border. Mexico claimed the border was the Nueces River, while the U.S. claimed it was the Rio Grande, much further south. This created a large disputed zone between the two rivers.

President James K. Polk sent General Zachary Taylor’s troops into this disputed territory. When Mexican soldiers fired on them, Polk claimed Mexico had "shed American blood upon American soil." Congress declared war in 1846. Critics, including Abraham Lincoln, questioned the war, but the desire for land drove the conflict forward.

Section 3

The Mexican Cession

The war ended with the U.S. victory and the signing of the Treaty of Guadalupe Hidalgo in 1848. Under this treaty, Mexico was forced to cede (give up) 55% of its territory to the United States.

This vast land acquisition is known as the Mexican Cession. It included present-day California, Nevada, Utah, Arizona, and parts of New Mexico and Colorado. While the U.S. achieved its dream of stretching to the Pacific Ocean, the acquisition of this new land immediately reignited the explosive debate over whether slavery would be allowed in the West.