World War I ended in 1918 and the "Spanish flu" pandemic petered out in 1920. The "Roaring Twenties" (or "Jazz Age") were bookended by two momentous events: the USA banned consumption of alcohol in 1920 (a law that was repealed only in 1933), and in 1929 the stock market crashed. There were other milestones: the population of the USA passed 100 million in 1920, and, for the first time in the history of the USA, more people lived in cities than in the countryside, and in 1920 the Constitution was amended to allow women to vote. The economy expanded rapidly between 1922 and 1929, and the economic engine was truly the movement to the city that turned the USA decisively towards industry instead of agriculture. Meanwhile, the media (the radio and the record besides the press) made the country more uniform, with people in every city listening to the same music (like jazz) and dancing the same dances (like charleston). The media also created the consumption society through the boom in advertising, which made people all over the country buy the same things, like Coca Cola. The car became ubiquitous, with Detroit companies (mainly Ford and General Motors) producing the vast majority of all the cars made in the world, and the government passed the Federal Highway Act for building an interstate system of highways. During that decade, Los Angeles' population more than doubled from 577,000 to over 1.2 million, the fifth largest in the USA. Los Angeles' boom of the 1880s had been driven by land speculation. The boom of the 1920s was many booms fueling each other: oil, cinema, agriculture, tourism, real estate.

All of this happened while organized crime wove a network of corruption in City Hall. Former anti-corruption crusader George Edward Cryer was formally mayor of Los Angeles between 1921 and 1929, but in reality the political boss of Los Angeles was his campaign manager Kent Kane Parrot, who protected crime kingpin Charlie Crawford, owner of brothels and gambling houses, and, by extension the top bootleggers of the city, both born in Italy: Tony Cornero (who made enough money to launch two floating casinos off the cost of Los Angeles in the 1930s) and Albert Marco (who was finally jailed for murder in 1929). Crawford was killed by prosecutor Dave Clark in 1931 and Clark's trial became the trial of the decade. Crawford's widow Ella was rich enough to build an open-air shopping mall on the site of the shooting, the Crossroads of the World (1936), designed by Robert Derrah. Cryer's and Pattor's careers had already ended in 1929 with the election of John Clinton Porter, a member of the Ku Klux Klan and an ally of the fanatical radio evangelist Robert Shuler (not a great improvement for Los Angeles' politics). Albert Marco was not working for the Mafia. The Mafia of Los Angeles was run by Sicilian mobsters Joseph Ardizzone and Jack Dragna, who ran a string of speakeasies, gambling dens and extortion rackets.

Oil strikes continued throughout the 1920s: Huntington Beach in 1920, Santa Fe Springs in 1921, and so on. Each one generated a tent city that thrived for a few years. The Santa Fe Springs one generated the fortune of Alphonzo Bell, the son of cattle rancher and Occidental College co-founder James Bell, who used the money to buy the old Mexican land grant Rancho San Jose de Buenos Ayres, west of Beverly Hills, and created (in 1923) a new subdivision, Bel-Air, soon to become a favorite of Hollywood stars. In 1923 Alphonzo Bell purchased the Santa Monica Land and Water Company (the one created by Arcadia Bandini and John Jones) and created subdivisions north of Pacific Palisades, overlooking the Pacific Ocean all of them Mediterranean-themed, notably Castellammare, whose first home was the 35-room Villa Leon (1928), designed by architect Kenneth McDonald for wool tycoon Leon Kauffman, and Miramar Estates, developed by Arthur Weber and George Ley. In 1928 Harry Chandler's newspaper Los Angeles Times hired architect Mark Daniels, who had designed several homes in Bel-Air, to build the luxury 14-room Villa Aurora, a showcase for the latest electric and gas appliances (with wooden ceilings imported from Spain, and a fountain imported from Italy). It was a failure as nobody bought it until 1943.

There were other speculators/investors. In 1905 Abbott Kinney purchased a tract of beach and began building a commercial Venice of California, which he called Venice: a network of canals (with gondolas from Italy), Renaissance-style buildings, a boardwalk, a pier, shops and restaurants.

In 1922 Methodist Church’s local leader Charles Scott wanted to create a religious utopia and convinced his congregation to purchase Pacific Palisades, the land that had belonged to John Jones. They hired architect Clarence Day to lay out a town and the Olmsted Brothers to expand it. Nestled between Malibu and Santa Monica, it ended up appealing more to Hollywood stars than to devout believers. Will Rogers and Mary Pickford were the first celebrities who moved there. Scott turned out to be a savvy investor and in 1926 partnered with Robert Gillis to acquire a piece of the original Abbot Kinney development which, designed by Mark Daniels became Huntington Palisades.

In 1927 Will Levington Comfort, an author of Western novels such as "Somewhere South of Sonora" (1925), founded the Theosophical colony Glass Hive near Pasadena and a monthly journal that consisted of his own spiritual ruminations and of articles by the French-born composer and astrologer Dane Rudhyar (born Daniel Chenneviere), who had converted to Theosophy while in New York and moved to Los Angeles in 1920. The Glass Hive became also a refuge for avantgarde artists such as Agnes Pelton.

The new stars of Los Angeles architecture were the modernists. Chicago master Frank Lloyd Wright built the Hollyhock House (1921) for oil heiress Aline Barnsdall and the Ennis House (1924), which resembles a Mayan temple, for retailer Charles Ennis, both in Hollywood, and, in Pasadena, the Millard House (1923) for bookseller Alice Millard. Austrian immigrant Rudolph Schindler, employed by Wright for the Hollyhock House, built his own Schindler House in Hollywood (1922) and the Lovell Beach House in Newport Beach (1926) for physician Philip Lovell; and his former junior partner Richard Neutra, another Austrian immigrant, designed the Lovell Health House (1929) in Hollywood, a pioneering steel-frame house.

There were stories within stories about these architects and their wealthy patrons. Aline Barnsdall was a young Chicago socialite who inherited the fortune of her father, Theodore Barnsdall, the largest independent oil producer, when he died in 1917. She was a feminist, a bohemian, an anarchist sympathyzer and a single mother, and obsessed with producing experimental theater, first in Chicago and then in Los Angeles. In 1919 she purchased Olive Hill (an olive orchard) in Hollywood and ask Frank Lloyd Wright to design not just a house but an entire art colony centered around a theater. Wright was busy in Tokyo and delegated the construction of her Hollyhock House to Schindler. The rest of the compound was never built. Meanwhile, Philip Lovell (a Jew born Morris Saperstein) was an health guru who promoted natural "drug-less" cures. He had traveled around the country without founding disciples until he arrived in Los Angeles. Through a weekly column in the Los Angeles Times (Harry Chandler was one of his early patients) and a radio show, Lovell built a following of wealthy people. He married Leah Press, a teacher who applied the unconventional educational theory of John Dewey in the kindergarten she shared with Schindler's wife Pauline. Barnsdall hired Leah Press as a tutor for her daughter and introduced the Lovells to the Chicago architects. Leah’s sister Harriet and her husband Samuel Freeman commissioned Wright their home (1925) but then hired Schindler to complete it, and the Lovells commissioned Schindler their beach house. The Schindlers were also friends with other Chicago immigrants, like Edward Weston and Karl Howenstein, who had been hired as director of the Otis Art Institute in 1922.

Albert Walker and Percy Eisen (the son of Theodore Eisen) designed many of the high-rise landmarks: the twelve-story Wurlitzer Building (1923) for the piano manufacturer, billed as "the world's largest music house" (it contained a concert hall), the twelve-story National City Bank building (1924) on Spring Street, the twelve-story Fine Arts Building (1927) of artist studios and an exhibition hall, as well as the Hollywood Plaza Hotel (1924) in Hollywood and the Beverly Wilshire Hotel (1928) in Beverly Hills.

The Art Deco style spread after construction of the Central Library, designed by New York architect Bertram Goodhue but completed in 1926 after his death; of the Bullocks Wilshire building (1929), designed by Donald Parkinson (John's son); and of the new City Hall (1928), the tallest building of the city for 40 years,designed by John Austin and Albert Martin.

Inspired by the movie sets of Hollywood, many businesses commissioned ornate exotic-themed buildings and Los Angeles became a cacophony of extravagant shapes. Buildings became three-dimensional advertisements of Los Angeles' crazy world, like the Spadena House (1926) in Culver City, designed by Harry Oliver, or the Mayan Theater (1927) in downtown Los Angeles, designed by Stiles Clements, or the Tower Theatre (1927) in downtown, that launched the career of architect Simeon Levi, who went by the name Charles Lee. The Tower and the even more sumptuous Los Angeles Theater (1931), also designed by Lee, were built for Chicago theater tycoon Herman Gumbiner.

In the middle of the booming 1920s, Christine Sterling (born Chastina Rix) was the first resident to think about preserving the Mexican heritage of the city. In 1926 she allied with the Los Angeles Times' publisher Harry Chandler to save the Avila Adobe, the oldest remaining home, dating from 1818. Funded by Chandler, who sensed a business opportunity, she created Paseo de Los Angeles (aka Olvera Street) on the site where El Pueblo de Los Ángeles had been founded, a touristy recreation of a traditional Mexican street, a sort of open-air theme park.

No wonder that in 1920 the publisher John Coke Brasfield started a magazine devoted to Los Angeles' mansions and gardens: the Architectural Digest.

The 1920s were also the age of the luxury hotel: New York theater and hotel magnate Junius Schine built the colossal Ambassador Hotel (1919), designed by Pasadena architect Myron Hunt, which included the Cocoanut Grove theater famous for the Charleston dance, and New York hotel magnate John McEntee Bowman built the even bigger Biltmore Hotel (1923), the largest hotel west of Chicago, designed by the New York firm of Leonard Schultze and Fullerton Weaver.

Another piece of the old Rancho San Jose de Buenos Ayres, a large undeveloped land between downtown and the ocean known as Wolfskill Ranch, to the west of Beverly Hills, was purchased in 1919 by retail tycoon Arthur Letts, and in 1922 his son-in-law Harold Janss (Peter Janss' son) began the subdivision which would become Westwood in 1929. Letts died in 1923 and Janss sold land to the University of California for a new campus, UCLA, which in turn, when completed, made Westwood a fashionable neighborhood, further designed as Mediterranean-themed town by developers Harland Bartholomew and Deming Tilton.

Harry Chandler took over the Los Angeles Times in 1917 and turned it into the most successful newspaper in the western states. He used his wealth to invest in real estate from San Fernando Valley to Mulholland Drive in Hollywood.

San Pedro Bay had been selected as the official port of Los Angeles in 1897 but it took until 1909 for it to become part of the city. After the Panama Canal opened in 1914, the port of Los Angeles became a strategic piece of international trade. In the 1920s Los Angeles passed San Francisco as the busiest port of the West Coast.

William Randolph Hearst was perhaps the most bizarre character of the 1920s, of the era called the "Jazz Age" and the "Roaring Twenties". The son of the mining investor and now senator George Hearst, William had built a newspaper empire starting with his father's San Francisco Examiner (1887), for which he hired star journalist Ambrose Bierce, and especially the "penny paper" New York Morning Journal (1895), whose exaggerated sensationalism and passionate defense of the Cuban rebels in 1898 (leading the USA into war against Spain) had generated record sales. Hearst had also founded the Chicago Examiner (1900), the Los Angeles Examiner (1903), the Boston American (1904), and acquired the Atlanta Georgian (1912), the Washington Times (1917), the Detroit Times (1921), the Seattle Post-Intelligence (1921), the Washington Herald (1922), plus magazines like Cosmopolitan (acquired in 1905) and Harper's Bazaar (acquired in 1913). Hearst got even richer when he inherited his father's mining fortune upon his mother's death in 1919, a fortune which he used to build an eccentric 165-room palace (today known as "Hearst Castle") on his family's vast coastal ranch near San Simeon. He hired art expert Julia Morgan and imported antiques from all over Europe as well as exotic animals from all continents (it became the world's largest private zoo). Despite being located four hours north of Hollywood, movie stars flocked to Hearst's wild parties at his "castle", the ultimate exhibition of opulence, where alcohol was free despite the Prohibition (between 1920 and 1933 alcohol was illegal in the USA). In 1918 Hearst founded a Hollywood film production company, Cosmopolitan Pictures, specifically to promote his lover, the actress Marion Davies, and in 1925 Hearst gave her a romantic gift: the medieval St Donat's Castle in Wales, Britain. In 1925 Hearst even purchased an entire 12th-century monastery from Spain and had it shipped to New York. In 1929 Hearst also financed a 110-room Santa Monica villa for Davies, designed by the same Julia Morgan, a giant beach house that became famous for costume parties. Then the Great Depression of the 1930s took a toll on his business empire: in 1933 he had to mortgage his "castle" to Los Angeles Times' owner Harry Chandler and in 1937 he started selling his monumental art collection to pay debts. The dismantled Spanish monastery remained in New York until, after his death, some investors rebuilt the monastery and relocated it to Florida.

Even the universities were beginning to make inroads: in 1921 CalTech hired Robert Millikan, a physicist renowned for having measured the charge of the electron. Two years later he was awarded the Nobel Prize in Physics, a first for California (although for work done in Chicago) and he shaped one of the most important schools of Physics in the country. In between, Hale hired Edwin Hubble (in 1919), who in 1924 would discover that the universe extends beyond the Milky Way and in 1929 would discover that the universe is expanding.

Religious movements came with the Midwesterners. The Canadian-born Pentecostal evangelist Aimee Semple McPherson, the widow of a missionary who had died in China, and later divorced from both her second and third husband, built a reputation as a charismatic preacher in the world of the nomadic tent revivals, and in 1918 settled in Los Angeles. She collected enough money from her ecstatic followers to build the first megachurch of the USA, Angelus Temple (1923). She ran both a weekly magazine and a radio station (1924), becoming the first major radio evangelist and a national phenomenon, the West Coast's version of Maria Woodworth-Etter (who had died in 1924). When she disappeared in 1926, and was believed drowned, the whole nation held its breath. When she reappeared, thousands of fans celebrated, but many suspected that she had simply eloped with a secret lover. Her rival was the Methodist preacher Robert Shuler, another evangelist who launched his own radio station in 1926, who attacked her integrity.

In 1912 Albert Powell Warrington became the president of the American Theosophical Society and decided to found the Krotona colony in Hollywood. Christine Wetherill Stevenson, an heiress to a Philadelphia fortune, joined Krotona in 1918 and bought Hollywood land from the heirs of Ivar Weed. She realized her dream of becoming a playwright with religious plays such as "The Light of Asia" (1918), about the Buddha, and "The Pilgrimage Play" (1920), about Jesus. For the latter, which required 150 actors and some animals, she hired architect Bernard Maybeck to build an open-air theater, the Pilgrimage Theatre (1920). The play became a summer event and the theater a tourist attraction. Annie Besant, the world leader of the theosophists from her base in India, misunderstanding something that the scientist Ales Hrdlicka (founder of forensic anthropology) had written, decided that a new race was about to be born in California and therefore in 1927 purchased land in Ojai Valley and established a new Theosophical community, replacing the one in Krotona.

Citrus farming, cinema and the aircraft industry required land, thereby increasing the need to expand outside the crowded downtown. Los Angeles became the most spread-out metropolis in the world, a confederation of urban communities rather than an organic town.

In 1920 the USA already boasted one car for every 13 people and Los Angeles (the capital of the oil economy) had one car for every 5 people. For the record, Britain had 1 for every 228 and Germany 1 for every 1017. By 1927 (when Ford finally retired the Model T) there were more than 20 million cars in the USA (20 million potential tourists) and there was a car for every other person in California. In 1926 a 4,000-kilometer stretch of road connecting Los Angeles and Chicago was designated as Highway 66, soon to become a legendary name.

Alvah Warren Ross is credited as inventing the shopping experience for car drivers. In 1920 he started developing a large retail center outside downtown that was specifically designed for cars, not pedestrians: the Wilshire Boulevard Center. It was New York's Fifth Avenue for people who drove instead of walking. In 1929 it became known as the "Miracle Mile" of Los Angeles.

The car restored the personal freedom that had been lost in the age of the railroad.

Just before the Great Depression, Los Angeles was celebrating its status as a booming metropolis. In June 1929 Los Angeles inaugurated the Roosevelt Highway (today's Pacific Coast Highway) that allowed tourists to drive 80 kilometers of uninterrupted coastline through Santa Monica, Pacific Palisades and Malibu. William Randolph Hearst financed the first round-the-world journey by an aircraft, Hugo Eckener's dirigible Graf Zeppelin: it was also the first aircraft to fly nonstop across the Pacific Ocean, taking off from Tokyo and landing in Los Angeles (to please Hearst) in August after a 79-hour flight.

At the same time, Los Angeles became famous for earthquakes after the Santa Barbara earthquake of 1925 that killed 13 people and the Long Beach one of 1933 that killed more than 100 people.

Meanwhile, further south, San Diego was becoming the site of a large naval base (inaugurated in 1922) which became one of the most strategic during World War I.

This was now largely a story about immigrants form other parts of the USA, not a story about foreign-born immigrants: the foreign-born population of the USA began to decline after the passage of the tough "Emergency Quota Act" of 1921 and the "Immigration Act" of 1924, which limited the number of immigrants through a national origins quota.

The 1920s witnessed the largest domestic migration in the history of the entire continent: more than 100,000 people flocked to southern California every year.