The Mexican–American War (1846–8) was ended by the Treaty of Guadalupe Hidalgo. By this treaty, according to Wikipedia, Mexico ceded California and "a large area comprising roughly half of New Mexico, most of Arizona, Nevada, and Utah, and parts of Wyoming and Colorado." Texas is conspicuous by its absence from this list.
Wikipedia also provides this map, showing "the Mexican Cession agreed with Mexico", which includes all of California, Arizona, New Mexico, Texas, Nevada, and Utah, plus about half of Colorado as well as parts of Wyoming and Kansas.
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