As Commander in Chief of the Army and Navy I have
directed that all measures be taken for our defense.
But always will our whole Nation remember the character
of the onslaught against us. No matter how long it may
take us to overcome this premeditated invasion, the
American people in their righteous might will win through
to absolute victory. I believe that I interpret the will of the
Congress and of the people when I assert that we will not
only defend ourselves to the uttermost but will make it very
certain that this form of treachery shall never again
endanger us.
Hostilities exist. There is no blinking at the fact that our
people, our territory, and our interests are in grave danger.

-President Franklin D. Roosevelt, Joint Address to
Congress, December 8, 1941

To what significant event did Roosevelt refer in this excerpt?