I think that a couple of arguments need to be addressed in order to fully assess the American actions in light of the attitudes leading to the war. The first would have to be that the modern understanding of Hitler and the Holocaust might not be entirely applicable to the time period. The easiest critique of the American isolationism prior to the war is to wonder how America could be silent to the threat of Hitler and the Holocaust. The problem with this argument is that it presumes a modern understanding of a historical and temporal reality. When we now use "Hitler" and "The Holocaust," it is based on the historical understanding of what the Holocaust actually was and what Hitler actually represented. It might be a bit unfair to be able to use the lens of historical reflection and apply it to the temporal context of what was happening. This is not to excuse isolationism in the face of pure terror, but rather to understand the political calculations of the time. Simply put, there was no vocabulary or patterns of recognition to effectively express what Hitler was and what he was doing. We now have that. We did not have that in the 1930s.
Another argument that has to be used is the reality of the Great Depression. Again, this is not to excuse the policy directives of the time, but the sincere understanding was that nothing like the Great Depression had been seen to that point. There had not been a period as intense as the Great Depression because there had not been a period of massive economic fluctuation like the 1920s. When the Great Depression hit, complete with unemployment and poverty on a widescale that had never been seen before, it was a reality that gripped America. The lack of economic prosperity or even economic hope held Americans of all background and narratives in the most tight of manners. This helped to focus and calibrate policy initiatives towards alleviating this condition, one that prevented any full reflection of what was happening in Europe. In this light, it becomes clear that one has to include the grip of domestic economic policy in precluding a full understanding of what was happening in Europe.
Finally, I think that the isolationism that was driving American policy prior to the outbreak of World War II came from a sincere attempt to wish to avoid World War I. Few can overstate the impact of World War I on America. Even though America entered the conflict later than other nations, the loss of life and the belief that involvement in European affairs can only lead to bad things for America drove American isolationism. In the final analysis, American isolationism and the policies that reflected this prior ot the outbreak of World War II were carved out of a condition that made clear that avoiding something like the First World War was the driving force behind American policy. There is little that one can say to blame the American isolationism coming from World War I for this position. Again, to indict them is to use the lens of history as a "Monday Morning Quarterback" and suggest an application of a modern metric to a temporal condition.
None of these arguments are meant to excuse American action. Essentially, American leaders knew Hitler was on the move and knew what was happening. Yet, to understand the political and legislative context of the time period are essential arguments to grasp in examining the role of American policy prior to the outbreak of World War II.