The Mexican–American War (1846–48) breaks out with the Whigs opposed to the war, and the Democrats supporting the war.

The U.S. army, using regulars and large numbers of volunteers, defeats the Mexican armies, invades at several points, captures Mexico City and wins decisively.

The Treaty of Guadalupe Hidalgo ends the war in 1848.

Many Democrats want  to annex all of Mexico, but that idea is rejected by southerners who argue that by incorporating millions of Mexican people, mainly of mixed race, would undermine the United States as an exclusively white republic.

Instead the U.S. takes Texas and the lightly settled northern parts (California and New Mexico)

The Hispanic residents are given full citizenship and the Mexican Indians become American Indians.

Simultaneously, gold is discovered in California in 1849, attracting over one hundred thousand men to northern California in a matter of months in the California Gold Rush.

A peaceful compromise with Britain gives the U.S. ownership of the Oregon Country, which is renamed the Oregon Territory.