California was more difficult. In 1845, Polk sent an envoy to Mexico City to offer Mexico as much as $30 million for present-day California, Nevada, Arizona, and New Mexico. The envoy, however, was never even allowed to make the offer and instead was ordered out of the country. Polk then ordered General Zachary Taylor and 1,500 troops to prepare to march to the Río Grande. Provoked, Mexican troops crossed the Río Grande and attacked Taylor in April 1846.
Immediately after he received the news, Polk “reluctantly” requested Congress to declare war. Congress granted Polk’s request after much debate. Whigs were particularly skeptical about who had actually started the war. Abraham Lincoln—then a congressman from Illinois—continually badgered Polk to identify the exact spot where the Mexicans had engaged Taylor. These “spot resolutions” gave the president a black eye and led many to believe that Polk had wanted and provoked the war himself.
The United States did not lose a single battle during the two-year war. Several months after the war had begun, John Frémont—an explorer and Polk’s agent in California—seized Los Angeles and accepted California’s surrender. With California secure, Polk then concentrated on campaigns in Santa Fe, Buena Vista, and Monterrey, and eventually captured Mexico City.
In 1848, the two sides signed the Treaty of Guadalupe Hidalgo, in which Mexico gave up nearly half of its territory to the United States (present-day California, Nevada, Utah, Colorado, Arizona, New Mexico, and Texas). The United States agreed to pay Mexico $15 million in exchange.
The Mexican War was a fairly popular war with the American people, for land-hungry settlers had been itching for more territory to farm and settle in the West. As a result, tens of thousands of American men enlisted in the army within the span of just two years. The spoils of war were demonstrable, as the size of the United States increased by a third.
For politicians, however, the war opened a huge can of worms. Accusations flew over who had actually started the war and why the war was being fought. Many Whigs (and historians) questioned Polk’s motives, believing that the war was more about California and manifest destiny than it was about Texas or U.S. security.