For the election of 1844, the Democrats nominated Speaker of the House James K. Polk on a platform supporting the annexation of Texas and demanding all of the Oregon Territory to the 54° 40' line. The Whigs, having formally kicked President John Tyler out of their party, Whigs selected Henry Clay (his third presidential bid). The new antislavery Liberty Party also nominated a weak candidate, mainly for show. In the end, though, Polk won 170 electoral votes to Clay’s 105; surprisingly, the Liberty Party stole just enough votes from Clay to tip the election toward the Democrats.
Tyler, concluding that Polk’s victory was a mandate from the American people to annex Texas, put the issue to a vote in both houses of Congress. Thus, in 1845, Congress officially annexed the Lone Star Republic. Mexico was outraged: they had refused to recognize Texas independence in 1836 and believed that the rebellious state would one day be reconquered. After the annexation announcement, Mexico withdrew its ambassador from Washington, D.C.
Then, a border dispute exacerbated the situation: whereas the United States claimed that Texas extended all the way south to the Río Grande, Mexico claimed that Texas was smaller, ending further north at the Nueces River. Both sides sent troops to the region, the Americans camping north of the Nueces and the Mexicans to the south of the Río Grande.
Polk went to the White House with a specific “to-do” list and accomplished all of his goals by the time he left. First, with the Walker Tariff of 1846, he reduced the tariff that had crept higher and higher since 1842. The new tariff set taxes on foreign goods at around 35 percent. Second, Polk reestablished the independent treasury that Martin Van Buren had created and that Tyler had decommissioned.
In addition, Polk wanted westward expansion, especially into Oregon and California. California had recently become a hot topic and prize in the West for its San Francisco Bay. However, acquisition of California would be difficult: it belonged to Mexico, which was not on good terms with the United States.
Acquiring Oregon was not difficult but did spark controversy. Recognizing that it could never win the population war in Oregon, Britain proposed giving the United States all of Oregon south of the 49th parallel rather than quarreling for the entire territory up to 54° 40'. The Senate agreed to the compromise, despite protests from many Americans who wanted the entire territory.
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 1 846.
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.
The war also renewed old debates on the westward expansion of slavery; some historians have claimed that in many ways the Mexican War was the first round between the North and South in the Civil War. On one side were the Whigs and a growing number of abolitionists in the North, who were adamantly against letting slavery spread. Against them stood the entire South and a majority of Democrats, who believed that expanding slavery was essential to the survival of their economy and society. This debate became the major issue—really the only issue—during the next decade until the outbreak of the Civil War.