It is important to realize that up until Pearl Harbor, and even after, public sentiment did not support our getting personally involved in the European war. Roosevelt's hands were tied and he was only able to send equipment and material aid rather than troops. All this changed when Hitler allowed himself to be swindled by his Japanese allies. Hitler wanted Japan to attack the Soviet Union, which would have probably been enough to crush them between both countries attacks. Japan had no intention of attacking Russia; preferring to consolidate their chain of islands and keep fighting China instead. Still, Japan intimated to Hitler that if he would declare war on the US then they would declare against the Soviets. Hitler fell for it and his declaration against the US untied Roosevelt's hands; allowing a much greater presence in Europe and ultimately Germany's downfall.
During the 1930s, US public opinion as well as several Senators questioned the validity of US involvement in the Great War. …
