The annexation of Texas marked a significant moment in the expansion of the United States, reflecting the era's prevailing belief in Manifest Destiny—the idea that the U.S. was destined to expand across the North American continent. After winning independence from Mexico in 1836, Texas existed as the Republic of Texas for nearly a decade. During this time, issues concerning its size, government, and relations with Native American tribes and Mexico dominated Texan politics.

The Republic faced numerous challenges, including economic instability and security risks from Mexico, which never formally recognized Texas independence. As settling American citizens in Texas increased, the appeal for annexation grew among many Texans who wished to bolster their security and economic prospects through U.S. statehood. The possibility of adding Texas to the Union stirred significant political debate, particularly surrounding the contentious issue of slavery, as Texas was a slave-holding territory.

In June 1844, U.S. President John Tyler, after considerable lobbying, signed a resolution for annexation, which gained traction in the U.S. Congress. Despite the political complexities, the resolution passed, leading to Texas becoming the 28th state of the United States on December 29, 1845. The annexation further aggravated tensions with Mexico, ultimately contributing to the outbreak of the Mexican-American War in 1846.