Why was Franklin Roosevelt such a successful president in terms of dealing with WWII?

I would argue that there were two major reasons why
Franklin Roosevelt was very succesful in dealing with the challenges of
WWII.

First, FDR was a very confident man who did not worry
much about what others thought about his decisions.  He was decisive in his
decision-making.  This is a very important trait in a leader, especially during war
time.

Second, FDR was a very good politician.  This was
especially important in WWII because of the fact that the US was part of a coalition of
allies.  FDR had to balance the demands of the various countries involved in the
alliance while also dealing with demands from US politicians and US military leaders. 
His ability to juggle these various demands was important in the Allies' ability to
fight the war effectively.