ON THIS DAY ·
Outbreak of the Mexican–American War
25 Apr–9 May 1846 — Mexican cavalry ambush a US patrol in the disputed strip, and Taylor wins at Palo Alto and Resaca de la Palma. Polk tells Congress that Mexico has “shed American blood upon the American soil,” and war is declared on 13 May.
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The moment in context
Expansion in the 1840s was a political programme as much as a migration, and President James K. Polk pursued it to the point of war. Its champions called it Manifest Destiny, the belief that Providence, meaning God’s plan, intended the United States to span the continent, much as some nations today claim a historic mission, and it carried a racial confidence that white Americans should rule. Texas, a slaveholding republic since it broke from Mexico in 1836, joined in 1845 by joint resolution, a simple vote of both houses like an ordinary law, after the Senate rejected an annexation treaty. Oregon was split with Britain at the 49th parallel, today’s Canadian border, and its map hatching vanishes in June 1846. When Mexico refused to sell California, Polk sent General Zachary Taylor into the hatched strip between the Nueces River and the Rio Grande, and a skirmish there in April 1846 let him present a war of choice as a war of defence.
From Mexican–American War, chapter 1 of The American Civil War, 1840–1877 (JUL 1848).
The place: Outbreak of the Mexican–American War — map and full story →
Teach this in 5 minutes
- Why it happened — Land hunger and an ideology of destiny. Americans were moving west faster than their government could follow, so Washington came under pressure to secure land its citizens had already…
- The turn — The Wilmot Proviso, 8 August 1846. The hinge of the decade came on the night a first-term Pennsylvania congressman added one sentence to a bill giving Polk $2 million to make peace.…
- What it changed — Free soil becomes a party. The proviso gave Northerners who opposed slavery’s spread a cause, free soil, meaning western land kept for free labour, that did not require them…
Then ask the room: Was the Mexican War a war for slavery? The argued answer is on the chapter page →
The atlas that shows it
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