​​Before California became a territory of America of course Spain was the main authority, but in fact it was part of the Mexicans too. Mexico has gotten independence and for that reason they attracted the attention of the American government that was interested in gaining that territory. This led to the Mexican- American war in the 1846-1848. The Americans got a securing control of California by the 13 of January 1847, after the battle of Southern California. After the war has resolved the land has divided between the Americans and the Mexicans. What is today California was the so-called Alta California the rest such as Baja Peninsula became Mexican, Nevada, Arizona, Colorado and the Utah became a U.S territories  whereas California became the 31st state on September 9, 1850.