Not the op, but since you seem serious, I think a better way of wording it would be, "spent centuries at war and attempted to genocide a few cultures". So, aside from the fact that most of the history of Japan is just waring states and almost never ending infighting, you can look into the Imjin War for Japan's first attempt at conquering the mainland in the 1500s. It would also be worth looking into their conquest and/or eradication of the Ainu, Emishi, Ryūkyū Kingdom.
Not the op, but since you seem serious, I think a better way of wording it would be, "spent centuries at war and attempted to genocide a few cultures". So, aside from the fact that most of the history of Japan is just waring states and almost never ending infighting, you can look into the Imjin War for Japan's first attempt at conquering the mainland in the 1500s. It would also be worth looking into their conquest and/or eradication of the Ainu, Emishi, Ryūkyū Kingdom.