 This , and other successful hit @-@ and @-@ run raids by American carriers in the South Pacific , showed that they were still a threat , although seemingly reluctant to be drawn into an all @-@ out battle . Yamamoto reasoned that another air attack on the main U.S. Naval base at Pearl Harbor would induce all of the American fleet to sail out to fight , including the carriers . However , due to the increased strength of American land @-@ based air power on the Hawaiian Islands since the December 7 attack the previous year , he judged that it was now too risky to attack Pearl Harbor directly . 
