Why did the US not enter World War II when it began in September 1939, and what social changes occurred on the US home front as a result?

The US did not enter the war in 1939 because it was under no treaty obligation to protect Poland in the event of an invasion. Many in the United States saw the entrance into World War I as a mistake, especially given how much the European powers squabbled over the spoils during the Versailles Peace Conference. There was also concern that the war was entered to fatten the bank accounts of government contractors. Many saw the Americas as the United States' sphere of influence and thought that the US had no business being in Europe. While some in the United States saw that Hitler was dangerous and would eventually have to be stopped, the voting public primarily wanted to work on domestic issues in 1939, especially stubborn unemployment rates which failed to drop below ten percent even during the New Deal.
The war affected the home front in many ways. Japanese Americans were interned immediately after the Pearl Harbor bombing in order to stop a suspected sabotage effort. Americans rationed and planted victory gardens. They also collected scrap metal and bought war bonds. Americans signed up for the war in droves and worked in munitions factories. The war was responsible for ending the Great Depression and ending sharecropping in the South, as poor whites and blacks took an active role in the war. Women also went to work in the place of their husbands who were fighting the war. Women also served support roles during the war. The war was responsible for demonstrating that segregation was a bad idea because Roosevelt declared that factories that received government contracts could not be segregated.