Japan Conquering the world - in Very Hard difficulty level
The conquest of the USA
Have you noticed that poor USA has just 4+1 non-garrison divisions at 1936, while Japan has more than 40? This means that you can gain about 100 IC within a year.
Simply carry all your land forces to Kwajalein, and carry out a surprise attack on Hawaii... From there, you can reach San Diego. Don't bother with California: it is a very rough terrain and no IC at all. If you have 2 spare divisions, you can take San Francisco and L. A., but your main aim must be the Eastern Shore. Just take Colon (2,500 kms from San Diego). Here there is a slight chance you may lose some precious ships (the most severe blow being the possible loss of some full transports) - probably during the whole campaign, you will have to save the most now.
From Colon, it's just 2 small jumps to New York. By the time you reach New York, your navy must have defeated US Navy several times (just stay together with your main force, and use your 2 Naval Bombers). From there, conquering the US is quite easy, just have some 6-10 divisions in reserve for the time when USA deploys some 3-6 militias and infantry. By 1937 summer, you can annex the USA.
Next step: America
This is an easy task: just follow the coast, and use your navy wisely. You will conquer America (except Canada) by the end of 1938.
A tough fight: China
When the Marco Polo bridge incident triggers, choose fight. That 1 Interventionism level worths the loss of Manchukuo and Korea. As soon as you have researched Armor type III, build as much as you can. With your victorious army, and your newly deployed >10 Armor divisions, you can defeat the Chinese by around early 1941.
Now you must make a severe decision: either attack the Allies or Soviet Union. When I first tried this strategy, it was mid-1942 when I finished with China, and first I attacked SU to help the Germans. But it was a long fight, and after all, the Germans took the fruit in the shape of Moscow...
Attacking the allies
This is an easy task, just don't rush for England, for it is filled with divisions now. Instead, initiate operations in India and in the Middle East. There you can destroy the major part of UK army, and easily annex UK.
Germany - or the Soviet Union
Here comes the biggest fight: if the Soviet Union conquered Germany, she must have more than 600 divisions by now. Not a big problem though, for by this time you must have about 50-70 armored divisions, and with the oil resources of the whole world to call on, you can beat them. Just use your superior navy, and encircle as many divisions as you can. After the fall of the Soviet Union, nothing stands between you - and a whole nice yellow world map!