5.3_The MEXICAN-AMERICAN War & Its PROBLEMS [APUSH Review]

The Mexican-American War is a significant event in U.S. history connected to the ideology of Manifest Destiny and U.S. Westward expansion.

The notion of Manifest Destiny asserted that the U.S. was destined to expand across the North American continent.
This period was characterized by a strong desire for the U.S. to grow westward, leading to the onset of the Mexican-American War.

James K. Polk: Became president in 1844, notably championing U.S. expansionism.
- Central campaign issues included:
- Annexing Texas.
- Settling disputes over the Oregon territory with Great Britain.
- Purchasing California from Mexico.

Texas was originally Mexican territory, populated by American migrants invited by the Mexican government to create a buffer against indigenous groups such as the Comanche.
Texas declared independence in 1836, becoming an independent republic for ten years.
The U.S. refrained from annexing Texas initially to avoid war with Mexico and due to debates over admitting new slave states.
In 1845, under Polk's presidency, Congress annexed Texas.

After being rebuffed in his attempt to purchase California for $15 million, Polk's administration sought to provoke conflict.
Border dispute over Texas:
- Texans claimed the border was the Rio Grande.
- Mexicans claimed the border was the New River.
In 1846, after sending American troops (led by General Zachary Taylor) across the disputed border, skirmishes led to fighting and ultimately a declaration of war.

Polk justified war on the grounds that American blood had been spilled on American soil, which was contested territory.
Congress declared war despite opposition from anti-war factions, including notable dissenters like Abraham Lincoln, who introduced the Spot Resolution to question where the conflict began.

The treaty resulted in the Mexican Cession, which included:
- Over half of Mexico's territory.
- The formal establishment of the U.S.-Mexico border at the Rio Grande.
- Acquisition of New Mexico and California in exchange for $15 million.
This acquisition represented the largest land acquisition in U.S. history, second only to the Louisiana Purchase.

The Mexican-American War reignited fierce debates over the extension of slavery into new territories.
- Wilsmont Proviso (1846): A proposal to ban slavery in lands acquired from Mexico, which, although voted down, stirred tensions.
- Southern Democrats had differing reactions to the Treaty:
- Some argued for further territorial expansion all the way to Mexico to ensure slavery's survival.
- Figures like John C. Calhoun expressed racial concerns about integrating Mexicans and indigenous peoples into U.S. citizenship.

Despite the notion that the newly acquired lands were sparsely populated, the area was rich with existing populations.
Initial relations between Anglo settlers and Tejanos (Mexican residents in Texas) soured:
- Many former Mexican citizens were disenfranchised and faced systemic racism.
- Anglos pressured Tejanos into assimilation and marginalized their presence.
Cortina Wars: Led by Juan Cortina, a Tejano landowner resisting Anglo encroachment, who sparked conflicts that lasted for decades despite initial defeat.

The Mexican-American War bolstered U.S. military prestige but escalated divisions over slavery that would deeply affect the nation leading into the Civil War.
Further discussion of the implications will continue in subsequent materials, particularly in connection to the intense debates over slavery in the coming years.