This week (May 11) in 1846, U.S. President James K. Polk asked Congress for a declaration of war against Mexico because, as he put it, “Mexico has … invaded our territory and shed American blood upon American soil.”
That was, to put it mildly, inaccurate. For years Mexico and the United States had been arguing over possession of the Texas territory, historically a part of Mexico, whose residents in 1846 were mostly Americans. Because those Americans had repeatedly petitioned Congress to annex Texas, Polk’s predecessor, President John Tyler, finally submitted a resolution to Congress annexing Texas, which Congress narrowly passed, infuriating the Mexican government. Then Polk added insult to injury by arguing that not only was Texas part of the United States, but its true border with Mexico was the Rio Grande River, not the generally acknowledged Nueces River. This increased the size of Texas, at Mexico’s expense, by another 150 miles to the south.
What is more, Polk previously had sent a military force under General Zachary Taylor into this disputed territory between the Nueces and Rio Grande rivers, which was such a provocation that Mexico promptly attacked Taylor’s force, spilling the “American blood” that Polk mentioned in his war message. American blood it may have been, but “American soil” it was not. Indeed, Polk’s claim to the Rio Grande border was based on a treaty with Mexico that its former leader, Lopez de Santa Anna, had signed while in a Texas prison, which explains why the Mexican government repudiated it.
But Mexico’s attack gave Polk the pretext to start his war, and to ensure that Congress went along, he attached his war declaration to a military appropriations bill, making it impossible for Congress to vote no. After all, U.S. soldiers had been attacked. Who would possibly vote against arming and supplying them?
The United States, of course, won the Mexican-American War, gaining Texas to the Rio Grande and also gaining the New Mexico and California territories, essentially completing America’s continental expansion to the Pacific. Which is why many historians consider Polk among our greatest presidents. He acquired for us the West.
But at a steep price. Polk’s “Yanqui imperialism” against Mexico so disgusted the rest of Latin America that they distrust America even today. In addition, his borderline-illegal machinations to start his war dangerously expanded the president’s war-making powers and set a bad precedent.
And finally, all that new land exacerbated the tensions between the North and South over the slavery issue, in particular whether Polk’s new territories would become free or slave states. That would lead inexorably to America’s bloodiest war, the Civil War, which — it is sometimes forgotten — was fought not over slavery, but over slavery’s expansion.