In 1821, Mexico had won its independence from Spain, and Texas became a part of the new Mexican republic. That same year Mexico allowed Moses Austin to bring American settlers into Texas. Moses died before seeing his son, Stephen F. Austin, bring 300 families to Texas. Even though Mexico prohibited slavery, they allowed it in Texas because they wanted to encourage the American settlers. Conflicts arose when the American settlers in Texas ignored Mexican laws.

Texans were upset when Mexico outlawed slavery and stopped immigration from the United States. In 1835, Mexico's president Antonio Lopez de Santa Anna sent 1,400 Mexican troops to Texas. A volunteer Texas army was formed to fight off Santa Anna's troops. The Texas army was defeated at the Battle of the Alamo. In April 1836, Sam Houston led the Texas Army on a surprise attack on Santa Anna near San Jacinto, where they defeated the Mexican army. This led to Texas's independence from Mexico.

Sam Houston became the first president of the Republic of Texas, and Texas became a state in 1845. Mexico refused President Polk's offer to buy California and New Mexico for $30 million and to establish the Rio Grande as the border between Texas and Mexico. In 1846, American troops led by Zachary Taylor marched across the Rio Grande and captured several Mexican cities. Battles were held in California and New Mexico, both were soon claimed by the United States.

In 1847, General Winfield Scott led an American army into Mexico City and captured the fortress Chapultepec. The war ended in 1848 with the Treaty of Guadalupe Hidalgo. Mexico agreed to the Rio Grande as its border with Texas and received $15 million for its territories. In 1845, journalist John O'Sullivan wrote about the right of the United States to claim new lands, which became known as manifest destiny.