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On this day in 1848, the United States and Mexico signed the Treaty of Guadalupe Hidalgo, ending a two-year war. The treaty set the southern Texas border at the Rio Grande and ceded Mexico’s northern provinces (which now include California and large parts of New Mexico, Arizona, Nevada, Utah and Colorado) to the United States in return for $15 million. In July of that year, the Senate approved the treaty’s ratification.
In discussions of constitutional war powers, it’s the start of the Mexican-American War that gets most of the attention. Congress’s move in 1845 to annex and grant statehood to Texas probably set the countries on a path toward eventual war. Nevertheless, the main constitutional controversy in most accounts of the war is that President James Polk pretextually manufactured a border crisis to pull Congress into declaring war over American blood spilled in the disputed region between the Nueces River and the Rio Grande (which the United States and Mexico, respectively, regarded as the proper national boundary). Polk had sent Army units into the contested territory, essentially baiting Mexican forces into firing the first shot. Polk had also already drafted a war declaration for Congress before word of the skirmish reached Washington in May 1846. When Polk told Congress that Mexico had killed American troops, his allies rushed the declaration through Congress.
The start of the Mexican-American War is only one of many important constitutional dimensions of the conflict, though, and I don’t think it’s the most interesting. I’m writing about this conflict on the anniversary of its end because its outcome—and the means necessary to achieve it—led to the war’s most noteworthy constitutional precedents.
