One of the sticking points when the IJN and the army were going back in forth on which option to take was that an attack on the Dutch East Indies would inevitably draw the U.S. into conflict with them. The Japanese navy knew even at that point that they would need to plan for a way to get the U.S. Navy knocked out so they would be able to maintain their gains.
One of my coming projects in the winter months is the find out what happened to all the gold that Japan Inc. stole from the occupied territories. One of the 'verboten subjects' amongst the academics I knew.