FDR's initial policy of isolationism during the early 1930's can be contributed to the economic crisis in the United States. When FDR became president in 1933, the US was in the midst of the Great Depression. This was the worst economic condition the US has ever experienced. In order to focus on helping the American economy recover through the use of his New Deal policies, FDR felt that getting involved in foreign relations would just cause more problems at this time.
This results in the US staying out of foreign affairs, specifically the rise of totalitarian leaders in countries such as Germany and Italy.
