Before the Japanese bombed Pearl Harbor, much of America did not perceive Hitler as a real threat. That is why the America First movement was so strong in the time before Pearl Harbor.
However, some Americans, notably President Franklin Roosevelt, did see Hitler as a real threat. Roosevelt felt that Hitler was a threat because he was so opposed to democratic forms of government. He felt that Hitler was bent on dominating the world and that Hitler would not be happy to have a strong and democratic US in existence since the ideals of democracy and those of fascism were so clearly opposed to one another.