Part A
The central idea in Roosevelt's "Address to Congress Requesting a Declaration of War" is:
The United States is already at war with the Japanese, since they attacked without provocation.
Part B
Roosevelt develops this central idea by:
Roosevelt describes recent attacks on the United States and in the Pacific that prove the Japanese want to engage in war.
This explanation underscores the sense of urgency and the justification for declaring war based on Japan's aggressive actions.