In the 1930s, the United States did not want to get involved in world affairs. They wanted to remain neutral and isolated. However, as Germany and Japan (and to some extent Italy) became more aggressive, President Roosevelt became less and less committed to neutrality.
The reason for this is that Roosevelt and some others saw the Germans and Japanese as a threat to American interests. They believed that a Europe that was dominated by Germany would be hostile to the US. They believed that Japanese expansion might endanger US outposts in the Philippines and Guam. As Germany and Japan became more aggressive in Europe and in China, the US became more and more concerned.
By making the US fear for its security, the Germans and Japanese eroded the US's commitment to neutrality and isolationism. Even so, of course, the US did not completely commit to war until it was actually attacked.
**adding onto the comment of pohpei397**
Up until the day of the Pearl Harbor attack, most Americans still
wanted to stay out of war, but afterwards the event sparked such
passion that it completely infuriated Americans into wanting to go to
This had been long in coming, as the U.S. had wanted to stay out of
war, but had still supported Britain more and more, and the U.S. had
been against the Japanese aggression but had failed to take a firm
stand on either side.
Finally, people decided that appeasement didn’t work against
“iron wolves,” and that only full war was needed to keep
the world safe for democracy and against anarchy and dictatorship.