The Mexican-American War

President James K. Polk wanted California long before any shots were fired. When he sent troops under Zachary Taylor into the disputed strip between the Nueces River and the Rio Grande, Mexican forces attacked, and Polk told Congress American blood had been shed on American soil. That gave him his war. Taylor pushed south and won battles at Monterrey, but the decisive stroke came from Winfield Scott, who landed an army by sea at Veracruz in March 1847. Instead of marching the long overland route, Scott struck at Mexico's main port, then drove inland along the old Spanish road straight to Mexico City, which fell in September 1847. That amphibious landing turned a grinding border campaign into a fast knockout.

The Treaty of Guadalupe Hidalgo (1848) ended the war with the Mexican Cession: Mexico gave up claims to Texas above the Rio Grande and sold the land that became California, Nevada, Utah, most of Arizona and New Mexico, and parts of Colorado and Wyoming, for 15 million dollars. A boundary quarrel had become a continental acquisition reaching the Pacific.

The war was never a unanimous cause. Whigs in Congress, including a young Abraham Lincoln, demanded Polk name the exact spot where blood was shed, suspecting he had provoked the clash to grab land. Henry David Thoreau went to jail rather than pay a tax he saw as funding an unjust war for slaveholders, and later wrote Civil Disobedience defending that choice. This fight over the war's purpose fed directly into the next crisis: how to handle slavery in the new territory, which erupted almost immediately with the Wilmot Proviso.