Why did the US enter WWII?

The only real answer to this is that the US entered WWII
because the Japanese attacked the US naval base at Pearl Harbor.  The US declared war
against Japan because of that action.

Many in the US
government, most notably President Franklin D. Roosevelt, had wanted to get into World
War II before the Japanese attack.  However, there had been no way to do so given that
the mood in the country was very much against entering the war.  That mood changed
immediately, however, once Japan attacked Pearl Harbor.  Americans wanted revenge then
and there was no essentially no opposition to going to war.  This was true a few days
later when the US declared war on Germany in response to Germany's declaration of war on
the US.

So, the US may have had other reasons for wanting
to go to war, but the Pearl Harbor attack was what actually caused the country to enter
the war.