5.3 Mexican American War

Period: 1844 - 1877
Historical Context: By the Election of 1844, Texas had been independent for nearly a decade.

Southern Democrats' Support for Annexation: Many Southern Democrats increasingly supported annexing Texas due to its agrarian potential.
Key Political Figures: The Democratic Party passed over Martin Van Buren to nominate James K. Polk, an ardent expansionist who aligned with Southern Democratic interests.
Whigs Response: The Whig party, represented by figures such as Henry Clay, opposed Polk's expansionist policies.

Election Result: Polk won the presidency with an electoral vote of 170 to 105, interpreting the narrow victory (a margin of 38,000 votes) as a mandate for expansion driven by manifest destiny.
Annexation of Texas: By December 1845, Texas became the United States' 28th state.

Manifest Destiny: This ideology was the primary driver leading to conflict with Mexico, encapsulating the belief that the U.S. was destined to expand from the Atlantic to the Pacific oceans.
Aggressive Expansionism: Polk's desire to acquire California and New Mexico intensified tensions with Mexico.
Slidell's Mission (1845):
- Objective: Polk sent John Slidell to negotiate the acquisition of California and New Mexico and to settle boundary disputes regarding Texas.
- Failure: The mission failed as no agreement was reached on the Texas-Mexico border. Mexico insisted on the Nueces River, while Slidell and Polk claimed the border lay further south at the Rio Grande.

Boundary Dispute: The U.S. claimed the Rio Grande as the Texas border, while Mexico claimed the Nueces River further north.
Military Maneuvers: Polk, supporting the Texan claim, sent General Zachary Taylor and a small army to protect Texas against a possible Mexican invasion.
Initial Conflict:
- In April 1846, Taylor's troops crossed into disputed territory, leading to an engagement with Mexican forces.
Request for War: Following the skirmish, Polk requested Congress to declare war, resulting in a declaration on May 13, 1846, with significant majorities in both Senate (40 to 2) and House (174 to 14) votes.
- Polk's Justification: He stated, "the cup of forbearance has been exhausted… invasion of our territory… shed American blood upon American soil."

Support from Citizens: Public rallies and calls to arms were prevalent, encouraging patriotic citizens to enlist in the military efforts against Mexico.

Locations of Combat: The war was primarily fought on Mexican territory encompassing California, New Mexico, and Texas. Key military campaigns included:
  1. Gen. Stephen Kearney: Led 1,500 troops to capture New Mexico and California.
  2. John C. Fremont: Took northern California and declared it an independent republic known as the Bear Flag Republic, named for its bear emblem.
  3. Zachary Taylor: Commanded 6,000 troops, achieving victory at the Battle of Buena Vista (1847).
  4. Winfield Scott: Led 14,000 troops to significant victories in central Mexico, capturing Vera Cruz and Mexico City (1847).

Northerner's Concerns: Many Northerners feared the war would enable the expansion of slavery into newly acquired territories.
Accusations Against Polk: There were accusations that Polk misled Congress regarding the war's initiation and that the U.S. was the aggressor.
Regional Sentiments: As one moved northward in the country, opposition to "Mr. Polk’s War" increased.

Control of Territory: By February 1847, the U.S. had gained control over nearly all Mexican territories north of Mexico City.
Negotiation of Peace: By September 1847, military successes pressured Mexico into negotiations.

Terms of the Treaty:
  - Mexico recognized the Rio Grande as Texas's southern border.
  - Mexico ceded New Mexico and California to the U.S. in exchange for $15 million, known as the Mexican Cession.
  - Additional land in southern New Mexico and Arizona was purchased for $10 million in the Gadsden Purchase (1853).

David Wilmot's Proposal: Aimed to prevent slavery's extension into territories acquired from the war. Stated that slave labor would compete with white labor, which would negatively affect white settlers.
Political Outcome: The bill passed in the House dominated by northern states but failed in the Senate controlled by southern states.

Impact of the War on Sectional Tensions: The acquisition of territories such as California and New Mexico intensified existing divisions over slavery, leading towards civil war.
- Northern Perception: Viewed the war as a means to extend "slave power" nationwide.
- Southern Justification: Southern states believed that preserving and expanding slavery was crucial for their economy and society.

The war reinforced feelings of nationalism and the ideology of manifest destiny.
- Historian's Perspective: The acquisition of California aimed to enhance trade relations with China and Japan rather than merely being racially motivated.

The debates over slavery in new territories were intensified, and conflict with Native Americans escalated as white settlers moved westward.

Territorial Expansion: The United States significantly expanded its territory westward through military victory and diplomacy, raising crucial questions regarding slavery, the rights and futures of American Indians, and Mexicans in these newly acquired lands.
Social and Economic Effects: Increased conflict and interaction between the U.S. government and Mexican Americans, as well as American Indians, negatively impacted these groups' economic self-sufficiency and cultural identities.
Mexico's Struggles Post-War: The war stunted Mexico's economic development, leading to long-term instability.