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Causes and Effects of the Mexican-American War

May 6, 2025

Heimler’s History: Mexican-American War

Introduction

  • Focus on Unit 5 of the AP U.S. History curriculum.
  • Key question: What were the causes and effects of the Mexican-American War?

Causes of the Mexican-American War

  • Texas Independence (1836):

    • Texas declared independence from Mexico.
    • Tensions cooled post-independence, but issues remained.
  • Annexation of Texas:

    • Texans didn't want to remain independent; they wanted U.S. annexation.
    • Presidents Jackson, Van Buren, Harrison, and Tyler avoided annexation.
    • James K. Polk's election changed the stance; his campaign promised Texas annexation.
    • John Tyler began the annexation process, influenced by Polk’s election.
    • Mexico was unhappy about annexation.
  • Diplomatic Efforts:

    • Polk sent John Slidell to Mexico City:
      • Proposed purchase of New Mexico and California territories - rejected by Mexico.
      • Dispute over southern border: U.S. claimed Rio Grande, Mexico claimed Nueces River.
  • Military Actions:

    • Polk sent General Zachary Taylor to the Rio Grande, triggering conflict.
    • Mexican troops engaged, resulting in 11 American deaths.
    • Polk used this to justify a war, which Congress declared on May 13, 1846.

Effects of the Mexican-American War

  • Territorial Gains:

    • Treaty of Guadalupe Hidalgo (1848):
      • Established Rio Grande as the border.
      • Mexican Cession: Mexico ceded California and New Mexico for $15 million.
      • Gadsden Purchase added more territory in 1853.
    • Mexico lost over half its territory to the U.S.
  • Wilmot Proviso:

    • Proposed amendment to prohibit slavery in new territories.
    • Highlighted tensions over slavery.
    • Although defeated, it underscored economic vs. moral reasons for free soil.
    • Seen as an early factor leading to the Civil War.
  • Impact on Inhabitants:

    • Non-American residents (Mexicans and Indians) largely stayed.
    • Treaty of Guadalupe Hidalgo promised U.S. citizenship to Mexicans.
    • Indians were excluded from citizenship, facing discrimination.
    • Civil rights assaults including voter and educational discrimination.

Conclusion

  • Importance of understanding the causes/effects for exams.
  • Encouragement to study and subscribe for more educational content.