Japan wanted a sphere of influence in Asia,and wanted to show the European powers that had colonies there (British, Dutch, French, etc)that a new "era" had dawned and that the Asian people were not going to accept Western rule. That was the most promulgated propaganda used. The major reason was more economic based as Japan was in great need for raw materials (as Japan was poor in this area). Japan was especially lacking in petroleum and SE Asia also provided rubber, rice, and a market to sell finished goods to.