The American frontier expanded in 1819 with the designation of Arkansas as a U.S. Territory. People pushed farther south and west, settling in Spanish-governed Texas. After Mexico won its freedom from Spain in 1821, Texas became a Mexican territory. Texas settlers agreed to join the Catholic Church and abide by Mexican law in order to live there.

By the 1830s, Texas’ growing American population unnerved Mexican leaders. In March 1836, Texans declared their independence from Mexico, and General Antonio López de Santa Anna set out to put down the rebellion. After a short but bloody war, Texas’ independence was established. Santa Anna, captured at the end of this struggle, was forced to sign a treaty ending the war and recognizing the independent republic south to the Rio Grande, but the Mexican government never acknowledged that treaty.

The two countries continued to fight, and Mexican officials knew that the U.S. was helping the new republic. When U.S. leaders suggested annexing Texas, relations grew worse. James K. Polk won the 1844 U.S. Presidential election while advocating the annexation of Texas and westward expansion to fulfill the country’s Manifest Destiny.