There is no single moment that can be considered to be the moment when the U.S. shifted from a rural to an urban nation. This was a gradual process that began after the Civil War and up to the twentieth century. However, if we had to point to a particular moment, we might point towards 1920, as this was the year in which more Americans were living in cities than in the countryside for the first time.
One of the innovations that had the greatest influence on this shift was the introduction of the assembly line. This development, along with interchangeable parts, made production in factories extremely efficient. As factories became more profitable, they became more common, and this meant that more jobs were available in cities. This led people to leave their towns and head to the city in order to find better opportunities for work.
