The reason for the American people didn't wanted to join in the war was from a document saying that the US will not get involve and that the Europeans will not colonize in the
US. Also that the US will not gain anything from being apart of the war. Instead we will lose people resources and loads of money.
The ideas that were put out that made US go to war was that the US found out that the germans were killing tons of people. Also that our Allies were being under attacked and the fear that were next (American People). That was enough to get the people motivated but not congress. the final straw was [Pearl Harbour]. After the incident with Pearl Harbour the United States of America decide to step in to the fight against the germans.
In the End Roosevelt had to convince millions of people. Also Congress to be able to go fight against germany. He have successfully convince the american people but not congress since congress wanted to do the right thing and stay out of the europe.