The japanese defense perimeter idea here involves taking a lot of these islands, building air fields on them. And then creating a sort of web of defence that makes counter attacks by the allies trying to retake their territory a brutally costly when the second warl war breaks out. If you are the allied strategists, this is a difficult strategic nut to crack, if you think about it,. and that's exactly what the japanese are hoping for.
Japan's rising sun goes supernova and engulfs a huge area of Asia and the Pacific. A war without mercy begins to develop infusing the whole conflict with a savage vibe.