Question

why does roosevelt use repetition in his request for a declaration of war?
to highlight his own credibility
to provide alternative courses of action
to compare the experiences of countries
to emphasize the threat posed by japan

Explanation:

Repetition is a rhetorical device. In the context of a war - declaration request, it is used to draw attention and stress important points. Roosevelt would use it to emphasize the gravity of the threat from Japan, making the case for war more forcefully.

Answer:

to emphasize the threat posed by Japan