The main reason why it was difficult for the US to enter World War II was World War I.

After WWI, the majority of the American people were very unhappy with the fact that the US had been involved in the war at all.  They felt that the war had not been in the national interests of the United States.  They felt that the US had been dragged into a war that was not its affair, largely for economic reasons.  Some felt that the US had only gotten into the war because it was to the benefit of large corporations.

Because so many Americans felt this way, a series of laws were passed in the 1930s to make it harder for the US to get involved in a war.  These were called the Neutrality Acts.  These acts did not prevent the US from entering a war, but they did make it illegal to trade with countries that were at war.  This made it clear that Americans did not want to get into another foreign war.

When WWII broke out, there was huge pressure on President Roosevelt to stay out of the war.  This lasted until the Pearl Harbor attack.