US Diplomatic Efforts to Avoid War with Mexico (1845–1846)

How the US Tried — and Failed — to Avert the Mexican-American War

Before the outbreak of war in May 1846, the Polk administration pursued a dual-track strategy: **aggressive diplomatic pressure** combined with **military provocation**, while publicly framing efforts as peaceful negotiation.

1. Diplomatic Mission: John Slidell (Nov 1845 – Mar 1846)

• Mission: Secret envoy to Mexico City with **three-tier offer**:
  (1) **Recognize Texas annexation** (Rio Grande border).
  (2) **Forgive $3.25M US citizen claims** vs Mexico.
  (3) **Purchase New Mexico ($5M) & California ($25M)**.
• Outcome: **President José Joaquín de Herrera** willing to receive Slidell but **overthrown by General Mariano Paredes** (nationalist, anti-negotiation) Dec 1845. Paredes **refused credentials**. Slidell rejected. Mission failed.

2. Earlier: Tyler's Annexation Treaty (1844)

• President **John Tyler** negotiated **annexation treaty** with Texas (April 1844).
• Included **US assumption of Texas debts**, **boundary at Rio Grande**, **purchase of NM/CA** implied.
• Senate rejected (2/3 needed; sectional split over slavery). Tyler then used **Joint Resolution** (simple majority) March 1845 — annexed Texas *before* Polk inaugurated.
• Mexico **broke diplomatic relations** (not war) — viewed annexation as *casus belli*.

3. Compromise Proposals (Congress & Press)

• Whigs (Clay, Webster, Lincoln): Opposed war. **"Spot Resolutions" (Lincoln):** Demand exact spot where US blood shed on US soil.
• Some Democrats: Wanted only Texas, not empire.
• Proposals: Arbitration, joint commission, recognize Nueces River border (Mexican claim), buy only San Francisco Bay.
• Result: **Polk ignored/opposed compromise.** Wanted California at any cost.

4. Military Provocation as "Diplomacy"

• Jan 1846: Polk orders **General Zachary Taylor** to move from Corpus Christi (Nueces) to **Rio Grande** (disputed territory) — **deliberate provocation**.
• Mar 1846: Taylor builds **Fort Texas (later Fort Brown)** opposite Matamoros — **act of war** per Mexico.
• Apr 25, 1846: Mexican cavalry ambushes US patrol (Thornton Affair) — **11 killed**.
• May 11, 1846: Polk tells Congress: **"Mexico has passed the boundary of the US, invaded our territory, shed American blood on American soil."**
• May 13: **Congress declares war** (Whigs vote for supplies, against war).

5. Nicholas Trist's Peace Mission (1847) — *After* War Started

• Sent with Scott's army to Mexico City.
• Negotiated **Treaty of Guadalupe Hidalgo** (1848) — got California, NM, Rio Grande border for $15M + claims.
• **Recalled by Polk** but stayed, finished treaty. Polk accepted.

Conclusion

The US **went through motions of diplomacy** (Slidell) but **simultaneously engineered a military crisis** (Taylor to Rio Grande) to force a war Polk believed was necessary for **Manifest Destiny** (California). The "avoidance" efforts were largely performative; the strategic decision was expansion.