Modern Texas was originally part of a larger territory, Coahuila y Tejas—one of the states of Mexico, which became a nation in 1821 after gaining its independence from Spain. After Texas gained its independence from Mexico, the Republic of Texas (1836–1845) allowed slavery, and as a result its admission into the Union deepened the divisions between North and South that led to the Civil War. Texas' entry into the Union also fed American ambitions to expand U.S. control of the continent and led to the Mexican War (1846–1848), which resulted in the U.S. acquisition of California and territories in the Southwest.

Coahuila Y Tejas

In 1821, Spanish colonial administrators in Texas granted Moses Austin authority to colonize American settlers. Austin died shortly thereafter, and his son, Stephen, received permission from the new Mexican government to continue his father's work. By 1835, 13,500 families had legally immigrated to...