The Americans that left the United States to settle in Texas agreed to
Speak Spanish
Become Catholics
Be loyal citizens of Mexico
Texas was a state in Northern Mexico and the former Americans living there were citizens of Mexico. They rebelled against the lawful government of Mexico and gained their independence in 1836. They were annexed by the United States in 1845 and became the 28th state.
Subsequently Arizona, New Mexico, Utah, Nevada and California were stolen from Mexico as a result of the Gadsden Purchase.