The idea of Manifest Destiny — that the United States had the God-given right to expand across the North American continent — was a popular and fervently held belief in the mid-1800s. The idea justified taking Native American territory, and it incited claims to even more land.

Tensions with Mexico coincided with America's quest for expansion. Mexico, which had just won its independence from Spain, had originally encouraged U.S. settlers in Texas, but its dictator, General Antonio López de Santa Anna, later banned further U.S. immigration. And when Texas declared its own independence from Mexico in 1836, Santa Anna marched to San Antonio with a force of 3,000 men to put down the insurrection. He surrounded 200 Texans, including Davy Crockett and Jim Bowie, at the Alamo, an old abandoned mission. Refusing to surrender, the Texans held firm for ten days, but the Mexicans captured the Alamo and killed its defenders.