What were the causes of U.S. entry into World War II?

The most immediate reason for the US's actual entry into
WWII was the Japanese attack on the US naval base at Pearl Harbor in what was then the
Territory of Hawaii.  The Japanese attack led to the US declaring war on Japan.  From
there, Germany declared war on the US and the US was fully
involved.

The Japanese attacked Pearl Harbor because they
wanted to gain an empire in Asia.  They felt that they needed an empire so as to have
their own supply of resources like iron and oil.  They felt the US would try to stop
them from gaining this empire and so they attacked Pearl Harbor to try to disable the US
fleet and prevent it from resisting as they took their Asian
empire.

Before that, Pres. Roosevelt had been getting the
US more involved in the war in Europe, but only by giving aid to the Allies.  He felt
that this was important because he felt that Hitler was a major menace to US
security.

So, the US started getting involved in the war
because FDR thought Hitler was a danger to the US.  But the US only actually entered the
war because Japan attacked Pearl Harbor.