The war of 1846 to 1848 in which the United States, provoked by its own government, defeated Mexico and seized roughly half of its territory — the present-day American Southwest and California. It carried the dream of continental expansion from sea to sea, made a conquered minority of the Mexicans who suddenly lived under American rule, and handed the United States a vast new land whose fate, slave or free, would help tear the nation apart in civil war fifteen years later.
In 1846 the President of the United States told Congress that Mexico had "shed American blood upon American soil." It was a war message, and it worked. But the soil he meant was disputed ground, claimed by both nations, where he had deliberately sent troops to invite a clash. A freshman congressman named Abraham Lincoln rose to demand the exact "spot" where American blood had been shed, and never got a straight answer. Two years later the United States held roughly half of Mexico, from Texas to California. The war that made America a continental power was, in the judgment of one of its own veterans, Ulysses Grant, one of the most unjust ever waged by a stronger nation against a weaker one. It is best understood not as destiny fulfilled but as a conquest chosen, and remembered very differently on the two sides of the border it drew.