Mexican-American War Begins (1845)

Fri Apr 25, 1845

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On this day in 1845, the Mexican-American War began after the U.S. seized Texas from the Mexican government, fulfilling President Polk’s imperialist campaign promise of annexing both Texas and California.

Tensions between either government had already been increasing, but the act that initiated the war was Polk ordering troops to occupy contested land between the Nueces River and Rio Grande. On this day in 1846, a 2,000-man Mexican cavalry detachment attacked a 70-man U.S. patrol under the command of Captain Seth Thornton in the contested land.

Polk asserted that American blood had been shed on American soil (which Abraham Lincoln called “a bold falsification of history”), and a formal declaration of war from the U.S. government soon followed.

The outcome of the war resulted in the U.S. acquiring control over Texas, California, and large parts of New Mexico, Arizona, Colorado, Utah, and Nevada in the Treaty of Guadalupe Hidalgo. An “All-Mexico Movement” in the U.S. opposed the treaty, demanding that the country annex the entire country of Mexico.