Final answer:
Isolationism in the U.S. was fueled by the past cost of war, the Great Depression's economic effects, the geographic distance, and possibly concerns over military preparedness, all contributing to a reluctance to enter another European conflict.
Step-by-step explanation:
Isolationism acted as a deterrent to U.S. participation in the war for several reasons. Americans were predominantly influenced by the desire to avoid another European conflict after the costly engagement in World War I. This sentiment, coupled with the economic turmoil of the Great Depression, amplified the public and political mood of non-intervention. Secondly, the distance between the United States and the European theater created a false sense of security, leading many to believe that geographic separation would keep them out of harm's way. Lastly, although not explicitly stated in the reference material, concerns over the strength of the U.S. military may have also been a factor in the nation's initial reluctance to engage with the powers of Germany and Japan.