I don’t have the entire Japanese navy down by the DEI. Usually, I end up splitting it into two major fleets; one protecting the transports and assisting in taking the DEI, and the other guarding home waters, particularly against the US fleet. Once you decide to take the DEI, unless UK or ANZAC attacked Japan first, it will be considered an unprovoked declaration of war against those two powers, in which case the USA can, and probably will, declare war on Japan. Therefore you have to leave some of your fleet to ward off the Americans, plus have 3 fighters at your airbase for scrambling to help the home fleet and a fair amount of infantry on the home island just in case the US Navy beats your home fleet.
I made that mistake once too. I had my Japanese fleet roaming all over the Pacific smashing the pitiful Allied presence and the US “sneaked” up with 2 or 3 transports full of inf/art and TOOK MY CAPITAL. I think this was a game of just Pacific 40 so the captured Major IC on Japan stayed Major. By the time I could get my navy back up there, US had built a bunch of defense and I couldn’t take Japan back. Lesson learned.