 Japan 's vulnerability to blockade was studied . Using information supplied by the Board of Trade and the naval attaché in Tokyo , the planners estimated that the British Empire accounted for around 27 per cent of Japan 's imports . In most cases these imports could be replaced from sources in China and the United States . However , certain critical materials for which Japan relied heavily on imports were identified , including metals , machinery , chemicals , oil and rubber , and many of the best sources of these were under British control . Japan 's access to neutral shipping could be restricted by refusing insurance to ships trading with Japan , and chartering ships to reduce the number available . 
