Britain's worst nightmare: Japan strikes British colonies in 1939, takes Ceylon and liberates India.

After 2 years fighting in China with heavy IJA casualties, Yamamoto convinces army, navy and government leaders that the expensive IJN can be used to acquire rapidly invaluable, poorly defended British and French territories much faster and at a lower cost than gaining large territories of little value in China, fighting against a poorly supplied, but huge army.

Yamamoto explains that the American isolationist movement is very strong and likely to become stronger if Britain & France are rapidly defeated in the east, so that the US is not likely to enter the war. Moreover, during the attack (which will certainly please Germany), Japan can ask Germany to threaten Holland with war if it declares war on Japan. Accordingly, Japan will only face weak British and French forces for the few months it will take to finish the campaign.

Yamamoto is ordered to prepare a detailed plan for the campaign. He already has it and presents it:

On day zero, Japanese troops arrive in merchant ships in Hong Kong, British Borneo, Malaya, Singapore, Mauritius, the Burmese oil port, Penang, Ceylon, Rabaul, Port Moresby, Rangoon, Hanoi, Hobart (Tasmania), Wellington (NZ), New Caledonia and Reunion. A destroyer and a light cruiser will also arrive to refuel in each port (carrying more troops and supporting the invasions with artillery). Submarines will be ready to start sinking warships in these ports and provide artillery support after using up their torpedoes, when the attack begins. When ports are seized, other ships will land with reinforcements and heavy cruisers will arrive to provide covering fire. Carrier planes will provide support in all necessary places. The initial operation will secure intact oil wells and refineries, vital for Japan and ports and airfields to reinforce and supply the invasions.
Japan will inform Holland that the war is limited to Britain and France offer an alliance with Holland to secure the DEI. Any attack from the DEI will result in its invasion by Japan.
Japan offers Thailand an alliance to liberate the territories stolen by France and Britain from Thailand.

The capture of Reunion, Mauritius and Ceylon will secure the Indian Ocean for Japan and exclude the allies. On Day 1 Japan will announce to the world the liberation of India, which will make Japan popular in the US. Ceylon will fall within weeks. Japan will induce India to sign an alliance with Japan, in order to invade other British colonies, Persia, the middle east, etc, together and share them. India cannot refuse the alliance, for Japan will be its only source of oil, steel, grain, etc, and trade partner. The millions of Indian troops will help Japan and Germany considerably. When Japan and India capture Persia and the midle east (expelling the British), Germany and Japan will be able to link forces, providing Germany and Italy with oil, rubber, tin, nickel, etc, 

The rapid allied debacle in the far and middle east and the large Indian army will probably induce the Italy, Turkey, Rumania, etc, to form the a coalition with India, Germany, japan and Thailand, securing victory and enabling the IJA's plan to invade China and the USSR successfully.

After seizing French and British territories and securing oil, Japan can invade at leasure even weaker and more valuable, but much larger DEI.