 Yamamoto 's primary strategic goal was the elimination of America 's carrier forces , which he regarded as the principal threat to the overall Pacific campaign . This concern was acutely heightened by the Doolittle Raid on 18 April 1942 , in which 16 U.S. Army Air Forces B @-@ 25 Mitchell bombers launched from USS Hornet bombed targets in Tokyo and several other Japanese cities . The raid , while militarily insignificant , was a shock to the Japanese and showed the existence of a gap in the defenses around the Japanese home islands as well as the accessibility of Japanese territory to American bombers . 
