Following the Treaty of Guadalupe Hidalgo (February 2, 1848) that ended the war, the westernmost portion of the annexed Mexican territory of Alta California soon became the American state of California, and the remainder of the old territory was then subdivided into the new American Territories of Arizona, Nevada, Colorado and Utah. The lightly populated and arid lower region of old Baja California remained as a part of Mexico. In 1846, the total settler population of the western part of the old Alta California had been estimated to be no more than 8,000, plus about 100,000 Native Americans, down from about 300,000 before Hispanic settlement in 1769.[59]

In 1848, only one week before the official American annexation of the area, gold was discovered in California, this being an event which was to forever alter both the state’s demographics and its finances. Soon afterward, a massive influx of immigration into the area resulted, as prospectors and miners arrived by the thousands. The population burgeoned with United States citizens, Europeans, Chinese and other immigrants during the great California Gold Rush. By the time of California’s application to the US Congress for statehood in 1850, the settler population of California had multiplied to 100,000. By 1854, over 300,000 settlers had come.[60] Between 1847 and 1870, the population of San Francisco increased from 500 to 150,000.[61] California was suddenly no longer a sparsely populated backwater, but seemingly overnight it had grown into a major US population center.

The seat of government for California under Spanish and later Mexican rule had been located in Monterey from 1777 until 1845.[46] Pio Pico, last Mexican governor of Alta California, had briefly moved the capital to Los Angeles in 1845. The United States consulate had also been located in Monterey, under consul Thomas O. Larkin.

In 1849, a state Constitutional Convention was first held in Monterey. Among the first tasks of the Convention was a decision on a location for the new state capital. The first full legislative sessions were held in San Jose (1850–1851). Subsequent locations included Vallejo (1852–1853), and nearby Benicia (1853–1854); these locations eventually proved to be inadequate as well. The capital has been located in Sacramento since 1854[62] with only a short break in 1862 when legislative sessions were held in San Francisco due to flooding in Sacramento.

Once the state’s Constitutional Convention had finalized its state constitution, it applied to the US Congress for admission to statehood. On September 9, 1850, as part of the Compromise of 1850, California was officially admitted into the United States as an undivided free state. Its status as a ‘free state’ prevented the expansion of slavery to the Pacific Coast, which was a foremost concern for the pre-Civil War US Congress. Within the state of California, Sep 9 remains as an annually celebrated legal holiday known as California Admission Day.

During the American Civil War (1861–1865), California was able to send gold shipments eastwards to Washington in support of the Union cause;[63] however, due to the existence of a large contingent of pro-South sympathizers within the state, the state was not able to muster any full military regiments to send eastwards to officially serve in the Union war effort. Still, several smaller military units within the Union army were unofficially associated with the state of California, such as the “California 100 Company”, due to a majority of their members being from California.

At the time of California’s admission into the Union, travel between California and the rest of the continental United States had been a time consuming and dangerous feat. Nineteen years afterwards, in 1869, shortly after the conclusion of the Civil War, a more direct connection was developed with the completion of the First Transcontinental Railroad in 1869. California was then easy to reach.

Much of the state was extremely well suited to fruit cultivation and agriculture in general. Vast expanses of wheat, other cereal crops, vegetable crops, cotton, and nut and fruit trees were grown (including oranges in Southern California), and the foundation was laid for the state’s prodigious agricultural production in the Central Valley and elsewhere.

Indigenous peoples under early American administration

Group of California indigenous people.

Under earlier Spanish and Mexican rule, California’s original native population had precipitously declined, above all, from Eurasian diseases to which the indigenous people of California had not yet developed a natural immunity.[64] Under its new American administration, California’s harsh governmental policies towards its own indigenous people did not improve. As in other American states, many of the native inhabitants were soon forcibly removed from their lands by incoming American settlers such as miners, ranchers, and farmers. Although California had entered the American union as a free state, the “loitering or orphaned Indians” were de facto enslaved by their new Anglo-American masters under the 1853 Act for the Government and Protection of Indians.[65] There were also massacres in which hundreds of indigenous people were killed.

Between 1850 and 1860, the California state government paid around 1.5 million dollars (some 250,000 of which was reimbursed by the federal government)[66] to hire militias whose purpose was to protect settlers from the indigenous populations. In later decades, the native population was placed in reservations and rancherias, which were often small and isolated and without enough natural resources or funding from the government to sustain the populations living on them.[65] As a result, the rise of California was a calamity for the native inhabitants. Several scholars and Native American activists, including Benjamin Madley and Ed Castillo, have described the actions of the California government as a genocide.[67]

20th century

Hollywood film studios, 1922

The “Birthplace of Silicon Valley” garage, where Stanford University graduates William Hewlett and David Packard developed their first product in the 1930s

Migration to California accelerated during the early 20th century with the completion of major transcontinental highways like the Lincoln Highway and Route 66. In the period from 1900 to 1965, the population grew from fewer than one million to the greatest in the Union. In 1940, the Census Bureau reported California’s population as 6.0% Hispanic, 2.4% Asian, and 89.5% non-Hispanic white.[68]

To meet the population’s needs, major engineering feats like the California and Los Angeles Aqueducts; the Oroville and Shasta Dams; and the Bay and Golden Gate Bridges were built across the state. The state government also adopted the California Master Plan for Higher Education in 1960 to develop a highly efficient system of public education.

Meanwhile, attracted to the mild Mediterranean climate, cheap land, and the state’s wide variety of geography, filmmakers established the studio system in Hollywood in the 1920s. California manufactured 8.7 percent of total United States military armaments produced during World War II, ranking third (behind New York and Michigan) among the 48 states.[69] California however easily ranked first in production of military ships during the war (transport, cargo, [merchant ships] such as Liberty ships, Victory ships, and warships) at drydock facilities in San Diego, Los Angeles, and the San Francisco Bay Area.[70][71][72][73] After World War II, California’s economy greatly expanded due to strong aerospace and defense industries,[74] whose size decreased following the end of the Cold War.[74][75] Stanford University and its Dean of Engineering Frederick Terman began encouraging faculty and graduates to stay in California instead of leaving the state, and develop a high-tech region in the area now known as Silicon Valley.[76] As a result of these efforts, California is regarded as a world center of the entertainment and music industries, of technology, engineering, and the aerospace industry, and as the United States center of agricultural production.[77] Just before the Dot Com Bust, California had the fifth-largest economy in the world among nations.[78] Yet since 1991, and starting in the late 1980s in Southern California, California has seen a net loss of domestic migrants in most years. This is often referred to by the media as the California exodus.[79]

During the 20th century, two great disasters happened in California. The 1906 San Francisco earthquake and 1928 St. Francis Dam flood remain the deadliest in U.S history.[80]