In 1790, only 5% of Americans lived in cities. Not even a fully century later, in 1880, that percentage had grown to 35%. By 1920, over half of Americans were city dwellers. This increase in urbanization was influenced by several factors. First and foremost, America was becoming an industrialized nation, a nation of factories and production. What's more, factories became more mechanized than ever as they started to incorporate innovations like assembly lines and interchangeable parts.
Factories, however, needed workers, and thousands of Americans needed jobs. The Civil War displaced many people, especially in the South, and they sought work in cities. Immigrants also settled in urban, industrial areas as they fled the famines and political upheavals in their own countries (especially in southern, central, and eastern Europe) and searched for their own version of the American dream. In fact, from 1870 to 1920, about 25 million immigrants arrived in the United States, and many, if not most, of them settled in cities.
Of course, the influx of population into American cities and into factory life created many challenges. Thousands of people packed into small areas, creating horrible living conditions, complete with overcrowding, poor sanitation, pollution, lack of proper food, and the spread of disease. Factories themselves were largely unsafe, and workers had few, if any, rights. They lived with constant danger, long hours, low wages, and abuse from supervisors.
All of these challenges led to social movements that tried to find solutions to urban problems and support city dwellers. The labor movement grew rapidly to address workers' struggles and push for shorter hours, higher wages, and better working conditions. People like Jane Addams sought to offer aid and improve living conditions for families packed into tenement houses. Political organizers sought changes in laws to address the challenges of city and work life, as well as the corruption of urban political machines. Indeed, the Progressive movement for social change grew up and flourished because of American urbanization, and its efforts, in time, led to improvement in the lives of at least some urban Americans.