Pearl Harbor,Hawaii was attacked on December 7,1941 at 8:00 A.M. by the Japanese military. On Demcember 8, U.S. President Franklin Delano Roosevelt asked congress for and received a declaration of war angainst Japan. On Demcember 11, Geremany and Itlay, allied with Japan,decraled war on the U.S. The United States had entered World War||. Japanese Admiral Isoroku Yamamoto conceived the Pearl Harbor attack and Captian Minoru Genda planned it. Two things inspired Yamamoto’s Pearl Harbor idea: a porphetic book and a historic attack. The book The Great Pacific War, written in 1925 by Hector Bywater, abritish naval athuority.