 Indonesian co-operation allowed the Japanese military government to focus on securing the large archipelago's waterways and skies and using its islands as defence posts against any Allied attacks (which were assumed to most likely come from Australia).The Japanese colonial rulers divided Indonesia into three separate regions; Sumatra was placed under the 25th Army, Java and Madura were under the 16th Army, while Borneo and eastern Indonesia were controlled by the 2nd South Fleet of the Imperial Japanese Navy (the IJN).The 16th Army and the 25th Army were headquartered in Singapore and also controlled Malaya until April 1943, when its command was narrowed to just Sumatra and the headquarters moved to Bukittinggi.