On this day in 1941, at around 1:30 p.m., President Franklin Roosevelt is conferring with advisor Harry Hopkins in his study when Navy Secretary Frank Knox bursts in and announces that Japan had attacked Pearl Harbor.The attack killed more than 2,400 naval and military personnel.n this day, in an early-morning sneak attack, Japanese warplanes bomb the U.S. naval base at Oahu Island’s Pearl Harbor—and the United States enters World War II. President Roosevelt and Secretary of State Cordell Hull knew a Japanese attack was imminent.