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how did the mexican american war start?
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Short answer: it began with the U.S. annexation of Texas and a disputed border that led to a clash between U.S. and Mexican troops in April 1846 — after which President James K. Polk asked Congress for a declaration of war.
More detail:
- Background: Texas had won independence from Mexico in 1836 but Mexico never recognized that independence. When the United States annexed Texas in 1845, it inherited the dispute over where Texas’s southern border lay. The U.S. claimed the Rio Grande as the boundary; Mexico insisted the border was the Nueces River, farther north.
- Expansion politics: President Polk and many in the U.S. wanted more territory (California and New Mexico); the idea of Manifest Destiny made territorial expansion politically powerful. Polk also tried diplomatic options (sending John Slidell to buy land), but Mexico refused to negotiate while angry over annexation.
- The immediate spark: In early 1846 Polk ordered General Zachary Taylor to move U.S. troops south to the Rio Grande (into territory Mexico considered its own). On April 25, 1846 a patrol under Captain Seth Thornton was attacked by Mexican forces in the disputed strip (the Thornton Affair). Polk reported that “American blood has been shed on American soil” and asked Congress to authorize force against Mexico.
- Outcome: Congress declared war on May 13, 1846. The conflict grew into a wider U.S. invasion of northern Mexico and ended with the 1848 Treaty of Guadalupe Hidalgo, in which Mexico ceded vast territory (including present-day California, Nevada, Utah and parts of several other states) and recognized the Rio Grande as the Texas border.
Interpretations differ: contemporaries and later historians debate whether the war was a defensive response to aggression or an act of U.S. expansionism and provocation. If you want I can give primary sources (Polk’s war message, Thornton Affair reports) or a short timeline.
More detail:
- Background: Texas had won independence from Mexico in 1836 but Mexico never recognized that independence. When the United States annexed Texas in 1845, it inherited the dispute over where Texas’s southern border lay. The U.S. claimed the Rio Grande as the boundary; Mexico insisted the border was the Nueces River, farther north.
- Expansion politics: President Polk and many in the U.S. wanted more territory (California and New Mexico); the idea of Manifest Destiny made territorial expansion politically powerful. Polk also tried diplomatic options (sending John Slidell to buy land), but Mexico refused to negotiate while angry over annexation.
- The immediate spark: In early 1846 Polk ordered General Zachary Taylor to move U.S. troops south to the Rio Grande (into territory Mexico considered its own). On April 25, 1846 a patrol under Captain Seth Thornton was attacked by Mexican forces in the disputed strip (the Thornton Affair). Polk reported that “American blood has been shed on American soil” and asked Congress to authorize force against Mexico.
- Outcome: Congress declared war on May 13, 1846. The conflict grew into a wider U.S. invasion of northern Mexico and ended with the 1848 Treaty of Guadalupe Hidalgo, in which Mexico ceded vast territory (including present-day California, Nevada, Utah and parts of several other states) and recognized the Rio Grande as the Texas border.
Interpretations differ: contemporaries and later historians debate whether the war was a defensive response to aggression or an act of U.S. expansionism and provocation. If you want I can give primary sources (Polk’s war message, Thornton Affair reports) or a short timeline.
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