American Blood on American Soil Causes of the Mexican-American War
- Rosie Jayde Uyola
- Nov 10, 2025
- 4 min read
Updated: Nov 19, 2025
Target: I can explain the differing perspectives on the cause of the Mexican-American War by analyzing a map and conflicting primary source documents.
Key Vocabulary
Manifest Destiny: The 19th-century belief that the U.S. was destined by God to expand west to the Pacific Ocean.
Disputed Territory: An area of land that two or more countries both claim to own.
Rio Grande vs. Nueces River: The two rivers that were the center of the border dispute. The U.S. claimed the Rio Grande; Mexico claimed the Nueces River.
Part 1: Do Now (5 minutes)
Directions: Read the prompt below and write a 5-8 sentence response.
Prompt: Imagine you and your neighbor disagree about where your property line is. You claim your yard extends to the big oak tree; your neighbor claims their yard extends to the fence, 20 feet onto "your" side. In a detailed paragraph, describe what might happen if you started building a shed in that 20-foot "disputed" area. What would your neighbor's reaction be? How might it turn into a serious fight? |
Sentence Starter: If I started building in the "disputed" area, my neighbor would probably react by... This could turn into a serious fight if...
Part 2: Analyzing the Sources
Directions: Analyze the three documents below and then answer the questions that follow.
Source 1: Map of the Disputed Territory (1846) (Description)
The map shows the southwestern United States. It displays the state of Texas as the U.S. claimed it, with its southern border on the Rio Grande river. The map also shows what Mexico claimed: that Texas's border was much farther north, at the Nueces River.
Between these two rivers is a large, shaded area labeled "Disputed Territory." This is the land claimed by both countries. The map shows that President Polk ordered U.S. General Zachary Taylor to move his troops into this disputed area, south of the Nueces and all the way to the Rio Grande.
Source 2: President James K. Polk, War Message to Congress (Adapted Excerpt, May 11, 1846)
Original Text | Simplified Text |
Mexico has passed the boundary of the United States, has invaded our territory, and shed American blood upon the American soil. | Mexico has crossed our border, invaded our land, and killed American soldiers on our own soil. |
We sent a diplomat to Mexico... But the Mexican government refused to see him. After this refusal, I ordered our army to advance to the Rio Grande River to protect our border... | We sent a diplomat to make peace, but the Mexican government wouldn't even meet with him. After they refused, I ordered our army to the Rio Grande to protect our border. |
Mexico has declared that a war has begun. They have crossed the Rio Grande and attacked our soldiers... The war now exists, and it exists by the act of Mexico herself. | Mexico has declared war. They crossed the Rio Grande and attacked our troops. War exists, and it's Mexico's fault. |
We must protect our territory and defend the honor of our country. | We have to protect our land and our country's honor. |
Source 3: Representative Abraham Lincoln, The "Spot Resolutions" (Adapted Excerpt, December 22, 1847)
Original Text | Simplified Text |
We... ask the President to provide this House with information: First: Show us the exact "spot" where the blood of our citizens was shed, as you claimed in your message. | We are asking the President to show us the exact "spot" where American blood was spilled. |
Second: Tell us if that "spot" was not, in fact, an area inhabited by Mexican citizens. | Tell us if that "spot" wasn't actually a place where Mexican citizens lived. |
Third: Tell us if that "spot" was not, in fact, land that Mexico had always possessed and controlled. | Tell us if that "spot" wasn't actually land that Mexico has always owned. |
...We are asking whether the soil on which this blood was shed was truly "our soil." ...We must know if this war was started on a false claim. | We are asking if that "spot" was really "our soil." We need to know if this war was started based on a lie. |
Analysis Questions
Directions: Answer question 1 on your own. Then, answer question 2 based on the source your teacher assigns you.
1. Based on the map (Source 1), why did the U.S. and Mexico both believe they were right about the border?
Sentence Starter: The U.S. believed it was right because it claimed the... as the border, but Mexico believed it was right because it claimed the...
2. (Think-Ink) Was the U.S. justified in going to war? Use evidence from either Source 2 or Source 3 to write your argument.
Sentence Starter: I believe the U.S. was (justified / not justified) because (Polk in Source 2 / Lincoln in Source 3) argues that...
Part 3: Exit Ticket (5 minutes)
Directions: Answer the following prompt in a complete paragraph (5-8 sentences).
Prompt: Based on today's documents, answer the Essential Question: Was the United States justified in going to war with Mexico? You must use at least one piece of evidence from President Polk (Source 2) and one piece of evidence from Abraham Lincoln (Source 3) to support your argument. |
Sentence Starter: I believe the United States was (justified / not justified) in going to war with Mexico. President Polk (Source 2) argued that... However, Abraham Lincoln (Source 3) challenged this by... In my opinion, the stronger argument is... because...
