What action prompted the united states to oficially enter wwii?

By 1941, Nazi Germany annexed nearly all of Europe escape for remnants of the Soviet Union and the whole of the UK. In East Asia, the Japanese were carving out an empire that included Korea, large chunks of China, and various holdings in South East Asia. The US was not fully convinced to enter the war until the Japanese launched a surprise aerial attack on the naval base of Pearl Harbor on December 7th, 1941. Because of an oil embargo and fear of US intervention in the Pacific to halt the Japanese territorial expansion, the Japanese attacked Pearl Harbor which held the majority of the US naval fleet. This immediately prompted the US to join World War II on the side of the Allies (Soviet Union and UK) against the Axis (Germany, Italy, Japan).