The changeover in California from Spanish to Mexican control in 1821 brought new laws, and a shift of power from missionaries to secular governors and powerful ranching families. The emigration of Mexican settlers, Russian colonists, American fur trappers and homesteaders, and European entrepreneurs introduced new social complexities. The arrival of European weeds, crops, and animals — especially cattle and horses — transformed California's natural spaces. When the US-Mexican War broke out in 1846, much of California was unrecognizable to native groups that had witnessed the coming of the Spaniards less than 80 years before.
Decline of the Mission System When the final California mission was built in 1823, they were at the height of their powers. Missions were at once churches, towns, schools, farms, factories, and prisons, often operated in conjunction with a nearby military presidio and agricultural pueblo. But the vast lands controlled by the missions made them a target of Mexican republicans who, after gaining independence from Spain in 1821, began calling for the privatization of church property. Secularization began in 1834; half of all mission lands to be turned over to local native groups.
The Rise of the Californios The decline of the missions allowed for the rise of ranching in California. To encourage agricultural development, the new Mexican government distributed more than 500 land grants to prominent families. Well-connected could secure grants for each family member, creating an elite class of rancheros who controlled hundreds of thousands of prime acres.
These families mainly raised cattle for an emerging hide-and-tallow trade with American ships. These elite Californios — as they became known — held themselves apart from non-land-owning Mexicans and natives.
New Ethnicities and Identities Just as they had for the mission system, native Californians provided most of the labor for the emergent ranching economy. Spanish-speaking natives intermarried with working-class Mexicans, blurring already complicated racial categories.
Laborers were bound to their ranches, with difficult working conditions and few alternatives. In response, some natives fled inland, joining mountain or desert groups and using their ranching knowledge to organize raids on livestock.The Europeans and Americans Arrive In the 1830s and 1840s, increasing numbers of Europeans and Americans arrived in California. Some, left their ships, became Mexican citizens, converted to Catholicism, and married into Californio families. Most American visitors to Mexican California portrayed it as a land of abundant resources underutilized by the "idle" Californios. " their racial "impurity" and Catholicism.
Manifest Destiny With its abundant natural resources and useful ports, California presented a particularly appealing territory for annexation.
Presidents as far back as Andrew Jackson had considered ways to take California, but it was James Polk who found a way to justify war. In April 1846, border skirmishes with Mexico in Texas gave Polk an excuse to put Manifest Destiny into action. The U.S.-Mexican War lasted less than a year and gave the United States undisputed control of California, Texas, Nevada, Utah, and parts of four other present-day states. California was now under American control. Its changes had only begun.
Source: Essay: 1821-1847: Missions, Ranchos, and the Mexican War for Independence
Calisphere.org at The University of California