Los Angeles, officially the City of Los Angeles and often known by its initials L.A., is the second-largest city in the United States after New York City, the most populous city in the state of California, and the county seat of Los Angeles County.
Mexican rule ended during the Mexican–American War: Americans took control from the Californios after a series of battles, culminating with the signing of the Treaty of Cahuenga on January 13, 1847. Railroads arrived with the completion of the Southern Pacific line to Los Angeles in 1876.Oil was discovered in the city and surrounding area in 1892, and by 1923, the discoveries had helped California become the country's largest oil producer, accounting for about one-quarter of the world's petroleum output. By 1900, the population had grown to more than 102,000, putting pressure on the city's water supply. The completion of the Los Angeles Aqueduct in 1913, under the supervision of William Mulholland, assured the continued growth of the city. In 1910, Hollywood merged into Los Angeles, with 10 movie companies already operating in the city at the time. By 1921, more than 80 percent of the world's film industry was concentrated in L.A.