Title: What US states once belonged to Mexico? How did they become part of the US? - Quora
Headings: 
What US states once belonged to Mexico? How did they become part of the US?
Content: Texas, Arizona, New Mexico, Colorado, Utah, Nevada and California. How? Well…
Mexico gained independence from Spain in 1821. This war drained Mexico out which contributed to further neglect its Northern territories. While Mexico was at war with Spain, thousands of white colonists, cattleman, adventures, and mercenaries, were coming from other parts of North America that had been already invaded and occupied looking for land and gold they could find and obtain “by any means.” In this process, whites perpetrated horrible crimes against Mexicans, because they believed in the concept known as “manifest destiny” which in practice meant to “kill everyone and take their lands”. Men and boys were taken by force out of their homes to be murdered, the women and little girls would be raped, murdered, their homes would be set on fire, and their animals would be slaughtered. “Texans” placed themselves in land plots, and later enlarged their population so that by 1835 there were more white foreigners than native Mexicans in the area, and called the stolen lands “Republic of Texas.” Up until 1843 California, New Mexico and other portions of the Southwest were internationally recognized provinces of Mexico, until the U.S. decided it wanted those provinces and declared war on Mexico. In 1844, then president James Polk confides to his Secretary of the Navy that a principal objective of his presidency is the acquisition of California, which Mexico had been refusing to sell to the U.S. at any price.