@Greenfox If I were to assume the author actually has a thought behind things, I would assume that japan is feeling immensely pressured by the loss of import and is amidst a resource and economic crisis. So they are probably aiming to have the nearest country be an ally with goodwill to be willing to protect them and give favorable trade deals and possible to merge with and share technology, while showing their "military prowess" to everyone while they still can. This way they can both ensure that people dont dare attack them, and the ability to demand ludicrous terms of surrender to alleviate the crisis from a hard shift (starve the empire so japan doesnt starve, demand land so they have areas to exploit for resources and so on). Noone will rebel, as everyone knows just what happens when japan considers you an "insurgent" (except, how do they expect that when they let noone flee the battle?).
But that is all just if I assumed author thought stuff through. Which I very much don't get the feeling of. Feels more like a case of "war crimes are edgy an cool!" and power fantasy.