Starting in 1940 (18 months before Pearl Harbor), the nation mobilized, giving high priority to air power. American involvement in World War II in 1940–41 was limited to providing war material and financial support to Britain, the Soviet Union, and the Republic of China. The U.S. entered officially on 8 December 1941 following the Japanese attack on Pearl Harbor, Hawaii. Japanese forces soon seized American, Dutch, and British possessions across the Pacific and Southeast Asia, except for Australia, which became a main American forward base along with Hawaii.

Answer this question, if possible (if impossible, reply "unanswerable"): What event led to America entering the war?
the Japanese attack on Pearl Harbor, Hawaii