What caused the US to declare war on the Axis powers of Germany Italy and Japan?

Answer 1

There was no single event, it was a chain of events.Here is the sequence of events:

1. US embargoed scrap metals and aviation gasoline sales to Japan.
2. Japan's attack on Pearl Harbor (partially for military reasons and partially in retaliation for the US trade embargo).
3. US declared war on Japan.
4. Hitler declared war on the US in support of Japan (this was a big mistake).
5. Mussolini declared war on the US in support of Nazi Germany and Japan.
6. US declared war on Nazi Germany and Fascist Italy.