The Mexican-American War was fought in the mid 19th Century between the United States and Mexico. The United States or at least those in charge at the time wanted the United States to reach from the Atlantic to the Pacific and many US citizens had settled in land that was not yet part of the United States. Many White Protestant Americans believed that God wanted them to take over land belonging to others who were not white Protestants, well naturally the White Protestants thought they were superior to the Roman Catholic Mexicans and pagans of all types. Now isn’t that similar to WASPISH Americans today? James K. Polk had just become US president and he supported the expansionists.
19th Century American Democracy was in no way democratic by modern standards as only white Men could vote while black people and Native Americans could legally be kept as slaves in some states and many white Americans thought it was morally right to inflict genocide on Native Americans. Those Americans who supported the War wanted to extend their partial democracy to New States but also many white people wanted to extend slave plantations to the new area, indeed some white settlers in Mexican territory had brought slaves with them. Americans who opposed slavery didn’t want to see slavery extended to the new territories.
The Mexican government was committed to Roman Catholicism and welcomed immigrants provided they converted to the Roman Catholic Church. Incidentally at that time the Roman Catholic Church opposed democracy. Attitudes in those Mexican territories which the United States later conquered were mixed, some people and some populations welcomed the United States armies while others fought the United States and/or rebelled after they had been conquered. After two years of bloody war Texas was firmly established as part of the United States while California and New Mexico became states. Fortunately the slavers didn’t win permanently as the American Civil War and the 13th Amendment ended slavery in the United States.