 The problem with enforcing a close blockade with ships was that warships loitering off the coast of Japan would be vulnerable to attack by aircraft and submarines . Blockading Japanese ports with small ships was a possibility , but this would first require the destruction or neutralisation of the Japanese fleet , and it was far from certain that the Japanese fleet would place itself in a position where it could be destroyed . A plan was adopted for a more distant blockade , whereby ships bound for Japan would be intercepted as they passed through the East Indies or the Panama Canal . This would not cut off Japan 's trade with China or Korea , and probably not with the United States either . The effectiveness of such a blockade was therefore questionable . 
