Chicago is the third most populous city in the United States. With over 2.7 million residents, it is the most populous city in the state of Illinois and the Midwest. The Chicago metropolitan area, often referred to as Chicagoland, is home to nearly 10 million people and is the third-largest in the U.S. Chicago is the seat of Cook County. Chicago was incorporated as a city in 1837, near a portage between the Great Lakes and the Mississippi River watershed, and experienced rapid growth in the mid-nineteenth century. Today, the city is an international hub for finance, commerce, industry, technology, telecommunications, and transportation, with O'Hare International Airport being the busiest airport in the world; it also has the largest number of U.S. highways and railroad freight. In 2012, Chicago was listed as an alpha global city by the Globalization and World Cities Research Network, and ranks seventh in the world in the 2014 Global Cities Index. As of 2012, Chicago had the third largest gross metropolitan product in the United States at US$571 billion. As of 2015, Chicago is the 7th most expensive city in the world according to a report released by global financial services company UBS.
The World War I period and the 1920s also saw a major expansion in industry. The availability of jobs attracted African-Americans from the Southern United States. Between 1910 and 1930, the African-American population of Chicago dramatically increased from 44,103 to 233,903. Arriving in the hundreds of thousands during the Great Migration, the newcomers had an immense cultural impact, called the Chicago Black Renaissance, part of the New Negro Movement, in art, literature, and music. Continuing racial tensions and violence, such as the Chicago Race Riot of 1919, also occurred. The ratification of the 18th amendment to the Constitution in 1919 made the production and sale (including exportation) of alcoholic beverages illegal in the United States. This ushered in the beginning of what is known as the Gangster Era, a time that roughly spans from 1919 until 1933 when Prohibition was repealed. The 1920s saw gangsters, including Al Capone, Dion O'Banion, Bugs Moran and Tony Accardo battle law enforcement and each other on the streets of Chicago during the Prohibition era. Chicago was the location of the infamous St. Valentine's Day Massacre in 1929, where Al Capone sent men to gun down members of his rival gang, North Side, led by Bugs Moran.