US His Manifest Destiny Part 3: Mexican–American War, Treaty of Guadalupe Hidalgo, and the California Gold Rush

Causation: Why the Mexican–American War Broke Out

• Historians constantly revisit causation, so interpretations shift with new evidence and fresh questions.

Older / “Traditional” Interpretations

• U.S. schoolbook view: Mexico started the war by crossing the Rio Grande and attacking U.S. troops near modern $\text{Brownsville}$.
  • Mexican administration supposedly anti-American, obstructed diplomacy, issued a warning that annexing Texas would be treated as war.
• Mexican nationalist view (19th–early 20th c.): U.S. aggression fueled by capitalist greed; conquest of California the real objective; war was an immoral land-grab against a weak, peaceful neighbor.

Newer / Revisionist Interpretations

• Recent Mexican scholarship: Mexican fault—leaders were cornered by domestic instability; not fighting would have toppled the government and invited revolution.
• Recent U.S. scholarship: U.S. fault—Polk tried to bluff/bribe/bully Mexico (offered $25$ million for CA & NM). When Mexican troops attacked, Polk’s bluff was called; now he had to fight.

Diplomatic Breakdown, $1845\text{–}1846$

• Election of $1844$ = popular mandate for Manifest Destiny: Polk promised to resolve Texas, Oregon, & obtain California.
• Mexico reacts: breaks diplomatic ties, recalls ambassador, expels U.S. envoy.
• Mexican politics unstable: Santa Anna ➔ Herrera ➔ hard-liner Paredes.
• Polk stations Gen. Zachary Taylor at Corpus Christi (U.S. claims border is the Rio Grande, Mex. claims the Nueces). Navy readies a blockade — classic prelude to war.
• Polk sends Rep. John Slidell to Mexico City with instructions:
  1. Mexican recognition of Texas at the Rio Grande.
  2. Purchase CA & NM for $25$ million.
  3. U.S. assumes American citizens’ claims vs. Mexico.
• Herrera too weak; refuses even to meet Slidell. Polk orders Taylor to advance to the Rio Grande (present-day Brownsville) hoping to provoke combat.
• Skirmish of $09\,\text{May}\,1846$: Mexican cavalry & U.S. dragoons clash in disputed zone. Two days later Polk asks Congress for war—"American blood shed on American soil."
• War declared $11\,\text{May}\,1846$. Northern Whigs denounce it as a slave-power conspiracy, but a slim national majority supports it.

Military Campaigns, $1846\text{–}1848$

Overview & Expectations

• First U.S. war fought on foreign soil.
• Europe predicts Mexican victory (bigger army, home terrain). Many Americans expect quick capitulation (racist assumptions of Mexican “inferiority”).

Northern Mexico / Southwest Theater

1. Gen. Stephen W. Kearny leaves Fort Leavenworth, captures Santa Fe with negligible resistance.
   • Why so easy? Distance from Mexico City ➔ de facto autonomy; trade ties via the Santa Fe Trail orient city toward the U.S.
2. Kearny takes Albuquerque, then marches toward California.
3. Capt. John C. Frémont crosses northern route, seizes San Francisco equally easily.
   • Local power held by Californio rancher elite—many intermarried with prior U.S. settlers.
4. Result: Entire North‐Mexico frontier falls swiftly.

Rio Grande & Interior

• Taylor defeats Santa Anna at Buena Vista.
• Gen. Winfield Scott executes America’s first major amphibious landing at Veracruz (minimal resistance), then marches inland.
• Resistance stiffens nearer Mexico City; still, superior U.S. artillery/ maneuver prevail.
• Young officers distinguishing themselves: Robert E. Lee and Ulysses S. Grant (foreshadowing Civil War).
• Mexico City captured $\text{Sept}\,1847$.

Casualties & Cost

• U.S.: $\approx 1{,}800$ KIA, $\approx 4{,}000$ WIA, $\approx 11{,}000$ disease/accident deaths.
• Mexico: $\approx 50{,}000$ total casualties.
• Financial cost to U.S.: $100$ million—four times Polk’s original purchase offer.

Treaty of Guadalupe Hidalgo, $02\,\text{Feb}\,1848$

• Negotiator Nicholas Trist sent with orders to buy TX, CA, NM for $15$ million (Polk now wants more land, less , later disavows Trist; Trist ignores recall, finishes treaty).
• Senate ratifies by 2∕3; public war-weary.
• Cession adds \approx 33\% of U.S. territory: present-day CA, NV, UT, AZ, NM, plus parts of CO & WY (Texas borders acknowledged at Rio Grande).
• Immediate political bombshell: Will slavery extend into the new Southwest?

Wilmot Proviso & Sectional Crisis

• Rep. David Wilmot (PA) introduces proviso 1846: "Slavery shall never exist in any part of territory acquired from Mexico."
• Passes House, fails Senate—debated two years; never law, but ignites fierce sectional rivalry.
• Southern shift: slavery is constitutionally protected property; Congress has no right to restrict it.
• Northern counter: Congress may bar slavery under Necessary & Proper clause.
• Democratic Party fractures into Northern vs. Southern wings.

Legal Status of Mexican Residents

• Traditional U.S. republican principle: free persons within U.S. jurisdiction become citizens (glaring exception = Native Americans).
• Therefore, Mexicans inside the ceded territory automatically receive U.S. citizenship (per treaty terms) effective \text{Feb}\,1848.

Military & Technological Rehearsal for Civil War

• Mexican War = "warm-up":
  • Future Civil War officers gain first combat experience.
  • New tactics (e.g., large-scale amphibious assault) & technologies tested.

California: From Remote Province to Bonanza

Early American Presence

• Since early 1800\text{s}$small community (≈$400\text{–}500) of American merchants, sailors, missionaries in San Francisco—goal: open Pacific trade with China & Japan.
• Oregon Trail bifurcation at Continental Divide: branch south → California.
• By 1848$≈$14{,}000 Americans living mainly in fertile Central Valley (agriculture attraction).

Gold Discovery & Rush

• Treaty ink barely dry when gold found near Sutter’s Mill (outside San Francisco) \text{Jan}\,1848.
• Legend: discoverer instantly shouted the news—"dumbest American ever" (classroom joke) because he squandered chance for secrecy.
• Gold Rush 1849:
  • End 1849$:$25{,}000 Americans in CA.
  • End 1850$:$100{,}000 Americans.
  • San Francisco explodes from 500$→$20{,}000 in four months.

Social & Economic Consequences

• Inflation: basic supplies scarce.
• Overcrowding, crime (theft, assault, murders).
• Dispossession of Mexican & Indigenous residents.
• Demographic: overwhelmingly young, single men chasing quick wealth—minimal interest in farming or community-building; hyper-individualistic ethos.
• Who profits? Merchants, saloon-keepers, prostitutes, gamblers, lawyers—not most miners.
• 1849\text{–}1863$extraction ≈$50$million (1860$)$=$\approx 5\times 10^{11}$(2025$).
• By 1860 easy placer gold exhausted; remaining ore deep—requires steam pumps, dredges → industrial mining by capital-rich Eastern corporations; small prospectors squeezed out.

Statehood Crisis, 1849\text{–}1850

• Population leap lets CA skip territorial phase (ordinarily need 60{,}000$registered voters; CA already$100{,}000).
• Drafts free-state constitution (no support for slavery—white miners unwilling to compete with enslaved labor).
• Applies for free state admission.
• South outraged; argues southern half lies below Missouri Compromise 36^{\circ}30'$line. Reminder: Compromise of$1820 legally applied only to former Louisiana Purchase, not Southwest.
• This becomes immediate Congressional crisis (precursor to Compromise of 1850).

Big-Picture Conclusions

1. Expansionism = central U.S. narrative; validates Frederick Jackson Turner’s Frontier Thesis (perpetual push West since colonial era).
2. By 1848$the U.S. has <strong>achieved Manifest Destiny</strong>—Atlantic to Pacific—<em>within$75$years</em> of founding (Jefferson predicted$100 yrs just for Louisiana).
3. U.S. emerges dominant continental power: Britain (in Canada) non-hostile; Mexico decisively beaten; Spain & France gone. External survival threats gone.
4. Internal threat rises: slavery expansion tears nation; Mexican War & Wilmot Proviso bring sectional antagonism to a boil, directly foreshadowing Civil War (begins 13$$ yrs later).
5. Mexican War provides military rehearsal & fosters leaders (Lee, Grant, et al.).
6. Economic impacts:
   • Southwest minerals, farmland.
   • California gold infusion boosts U.S. money supply, finance, and transcontinental migration.
7. Treatment of Mexicans/Indigenous peoples exposes contradictions in U.S. claims of egalitarian citizenship.

As your instructor closed: Manifest Destiny is complete; the next existential struggle will be an internal one.

End of Notes. “Get in some good study time, and go Runners!”