Officially, World War II began with Germany's invasion of Poland on 1 September 1939. It had initially been a war between France, Germany, and Britain. However, other major players - Italy, Russia, Japan, and the USA - joined in the fray not long after. But it was only two years later that the US became more involved in the war.
In the beginning, the government and the general public had not wanted any direct participation. Things might have been different if America had not been provoked to join in WWII. But in the end, it was inevitable for the US to take part in it and the attack on Pearl Harbor only served to ignite the public and hasten America's participation.
To learn more about the events that led to America's decision to participate in WWII, check out this article on History Extra.