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Unit 5

5.3 The Mexican-American War

3 min readmay 30, 2020


Caleb Lagerwey

Manifest Destiny goes south...but not too far south.
A preface to the Mexican-American War was the Texas War of Independence (1835-1836), where Texas became its own country for a few years after defeating Mexico. This war involved the Alamo and other famous battles like Goliad and San Jacinto (Period 3).


Pro-expansionists, largely Democrats, had long eyed Mexico and other sub-tropical regions for their ability to grow crops that could lead to the expansion of slavery and the associated Southern way of life. Mexico seemed like a prime target.
President James K. Polk was elected on a pro-expansion platform, and his placement of troops in the disputed territory near the Rio Grande River led to conflict with Mexico. 
The United States won after two years of battles and negotiations (see map below for an overview), but you largely just need to know the causes and effects of the war.
🎥 Watch: AP US History - Manifest Destiny



The Mexican-American War was an important event as it serves as a link between Manifest Destiny and the Civil War: it is a great example of how Westward expansion led to increasingly bitter and divisive debates over slavery in new territories.
The Wilmot Proviso was an unsuccessful addition to a bill to fund the US army during the war. It argued for a complete ban on slavery in captured territories, which, of course, only made tensions worse. It passed the House of Representatives—hello, Northern population advantage!—but failed in the Senate.
This showcased the necessity for Southerners to keep the balance of states in the Senate as their population totals fell behind those of the North: the Senate, which its equal representation for each state regardless of population, was the safeguard for the South’s interests as the North continued to add to its population advantage in the House and Electoral College.

Image Courtesy of Wikimedia

The United States won the war and annexed the territories north of most of Mexico’s population centers—mostly to avoid annexing Catholics and non-Whites—and thus gained parts of New Mexico, Arizona, Nevada, and California, the area known as the Mexican Cession after the signing of the Treaty of Guadalupe Hidalgo that ended the war. 
The Treaty promised citizenship for Native Americans and Mexicans, but was largely ignored: the Mexicans who were remaining in the newly-acquired territory often lost land and property in courts, since white settlers considered them foreigners despite treaty promises, or because of the pressure to sell. Chinese immigrants who worked in the goldfields also suffered discrimination. California was on its way to getting statehood, but the question of its being a slave or free state seemed years away until the California Gold Rush of 1849 forced the issue through the sudden influx of thousands of settlers. See 5.4 for the impact of this.
🎥 Watch: AP US History - Period 5 Review


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