Throughout the 1820s, Americans settled in the vast territory of Texas, often with land grants from the Mexican government. However, their numbers soon alarmed the authorities, who prohibited further immigration in 1830. In 1834 General Antonio López de Santa Anna established a dictatorship in Mexico, and the following year Texans revolted. Santa Anna defeated the American rebels at the celebrated siege of the Alamo in early 1836, but Texans under Sam Houston destroyed the Mexican Army and captured Santa Anna a month later at the Battle of San Jacinto, ensuring Texan independence.

For almost a decade, Texas remained an independent republic, largely because its annexation as a huge new slave state would disrupt the increasingly precarious balance of political power in the United States.  In 1845, President James K. Polk, narrowly elected on a platform of westward expansion, brought the Republic of Texas into the Union.  Polk's move was the first gambit in a larger design.  Texas claimed that its border with Mexico was the Rio Grande; Mexico argued that the border stood far to the north along the Nueces River. Meanwhile, settlers were flooding into the territories of New Mexico and California.  Many Americans claimed that the United States had a "manifest destiny" to expand westward to the Pacific Ocean.

U.S. attempts to purchase from Mexico the New Mexico and California territories failed.  In 1846, after a clash of Mexican and U.S. troops along the Rio Grande, the United States declared war.  American troops occupied the lightly populated territory of New Mexico, then supported a revolt of settlers in California. A U.S. force under Zachary Taylor invaded Mexico, winning victories at Monterrey and Buena Vista, but failing to bring the Mexicans to the negotiating table. In March 1847, a U.S. Army commanded by Winfield Scott landed near Veracruz on Mexico's east coast, and fought its way to Mexico City.  The United States dictated the Treaty of Guadalupe Hidalgo in which Mexico ceded what would become the American Southwest region and California for $15 million.

The war was a training ground for American officers who would later fight on both sides in the Civil War.  It was also politically divisive.  Polk, in a simultaneous facedown with Great Britain, had achieved British recognition of American sovereignty in the Pacific Northwest to the 49th parallel.  Still, antislavery forces, mainly among the Whigs, attacked Polk’s expansion as a proslavery plot.

With the conclusion of the Mexican War, the United States gained a vast new territory of 1.36 million square kilometers encompassing the present-day states of New Mexico, Nevada, California, Utah, most of Arizona, and portions of Colorado and Wyoming.  The nation also faced a revival of the most explosive question in American politics of the time: Would the new territories be slave or free?