The industrialization of America led to incredible population growth in urban centers; by 1900, 40% of Americans lived in cities.
As the United States industrialized in the 19th century, immigrants and workers from the countryside were housed in tenements.
Many political machines in cities were affiliated with the Democratic Party, which recruited new immigrants, particularly the Irish.
When cities grew in population, their problems with pollution and disease increased exponentially.
The World's Columbian Exposition in Chicago was an international fair whose grandeur symbolized emerging American exceptionalism.