When did the United States formally declare war on Japan, entering World War II?

The story behind the answer

December 8, 1941, is correct: the day after Japan’s surprise attack on Pearl Harbor (December 7, “a date which will live in infamy”), President Franklin D. Roosevelt asked Congress for a declaration of war. It passed almost unanimously—only Representative Jeannette Rankin voted no—bringing the United States formally into World War II in the Pacific. Within days (December 11) Germany and Italy declared war on the U.S., expanding America’s involvement to the European front.

To remember: attack on the 7th, declaration on the 8th. The other dates mark different WWII milestones—September 1, 1939 (Germany invades Poland, war begins in Europe), June 22, 1941 (Germany invades the USSR), and June 6, 1944 (D-Day landings in Normandy).