Why did many Americans oppose U.S. participation in World War II?
Having seen what World War I did to Europe and what it did in terms of killing and maiming so many American boys, opposition to involvement in foreign wars had taken deep root in the American public and in many powerful politicians and influential Americans. After the onset of the great depression, many Americans also felt that focusing on solving the economic malaise afflicting so many citizens should take priority over any concern for international affairs and entanglements.
Time and again Congress opposed measures that FDR wanted to take and they also took other steps to try and preclude any entanglements that would push the United States into being involved with the turmoil in Europe at the end of the 1930's.
Some of the same racial prejudice that existed in Europe and helped Hitler to power also existed in the United States and there were quite a few people who sympathized with the Nazi leader and had no desire to spend American lives and treasure to oppose him.
It took the attack on Pearl Harbor to turn a majority of Americans in favor of the war and even as public opinion shifted, there were certainly still quite a few Americans who remained in opposition to involving ourselves in World War II.