During the Progressive Era (roughly 1900–14), many U.S. leaders and citizens believed that the government should take an active role in protecting individuals, especially children, workers, and consumers. They wanted the government to be free to make laws that would, for example, limit the size of companies so that smaller businesses could compete or stop employers from hiring children to work in their factories. In fact, U.S. involvement in World War I (1914–18; the United States entered the war in 1917) could be seen as a large-scale application of this belief, because it was supposed to make the whole world "safe for democracy" (a common saying of the period).

But that extremely bloody, destructive war took a terrible toll on humanity. When it was over, many people in the United States began to call for an isolationist stance, meaning that the nation should...