The Japanese had already promised to grant Burma independence, but they reasoned that granting independence early would further motivate the Burmese into supporting the war effort.
Thus Burma joined Thailand and other Asian states under Japanese control to become part of the ‘Greater East Asia Co-Prosperity Sphere.’ It controlled 95% of the world’s rubber and all of the world’s production of quinine (used to treat malaria).
The Allies were working hard to create artificial substitute for both.