The 1904 World's Fair-the Louisiana Purchase Exposition-signaled the end of the Victorian period. The first two decades of the 20th century were the zenith of St. Louis's national prominence: the City hosted both the largest World's Fair to date, and the 1904 Olympic Games. The 1910 census showed St. Louis to be the fourth largest city in the country, behind New York, Chicago and Philadelphia.
Evolutionary factors in society brought great changes to the character of civic life, and the built environment as well. The period after the turn of the century is known as the Progressive Era, because of the widespread social movement to enhance the quality of American life, in the face of mass immigration, and industrialization. The Progressive Era had its most important impact upon St. Louis in the rise of city planning. Beginning with the Civic Improvement League, founded in 1901, and continuing with the City Plan after 1910, efforts were made to enhance the quality of housing; to beautify downtown and City parks; and to improve public transportation, recreation and cultural life. The most long-ranging effect of the Civic League and the Plan Commission was the 1918 zoning plan, which attempted to isolate industrial uses outside residential areas. St. Louis was second only to New York among the major American cities to adopt industrial/residential zoning.
The popularity of the automobile had an immense impact upon the character of St. Louis and its buildings. Aside from the development of new property types relating specifically to the automobile (filling stations, garages, parking lots), streets were widened, driveways became common, and houses and apartment buildings were constructed with internal garages.By 1900, the decline of the major Victorian architectural styles became apparent. Italianate, Second Empire and Romanesque Revival models were replaced by the rise of Beaux-Arts, Arts and Crafts, Tudor, and Georgian Revival styles, and by the streamlined forms of Art Deco and Moderne.