Claim: California is a U. S. state.

Alta California -LRB- Upper California -RRB- , founded in 1769 by Gaspar de Portolà , was a polity of New Spain and after the Mexican War of Independence in 1822 , a territory of Mexico . The region included all of the modern American states of California , Nevada , and Utah , and parts of Arizona , Wyoming , Colorado and New Mexico .   Neither Spain nor Mexico ever colonized the area beyond the southern and central coastal area of present-day California , so they never exerted any effective control north of the Sonoma area , or east of the California Coast Ranges . Most interior areas such as the Central Valley and the deserts of California remained in de facto possession of indigenous peoples until later in the Mexican era when more inland land grants were made , and especially after 1841 when overland immigrants from the United States began to settle inland areas .   Large areas east of the Sierra Nevada and San Gabriel Mountains were claimed to be part of Alta California , but were never colonized . To the southeast , beyond the deserts and the Colorado River , lay the Spanish settlements in Arizona.Chapman explains that the term `` Arizona '' not used in period . Arizona south of the Gila River was referred to as the Pimería Alta . North of the Gila were the `` Moqui '' , whose territory was considered separate from New Mexico . The term `` the Californias , '' therefore , refers specifically to the Spanish-held coastal region from Baja California to an undefined north .   Alta California ceased to exist as an administrative division separate from Baja California in 1836 , when the Siete Leyes constitutional reforms in Mexico re-established Las Californias as a unified department . The areas formerly comprising Alta California were ceded to the United States in the Treaty of Guadalupe Hidalgo that ended the Mexican -- American War in 1848 . Two years later , California joined the union as the 31st state . Other parts of Alta California became all or part of the later U.S. states of Arizona , Nevada , Utah , Colorado , and Wyoming .