That's what they and the Western governments legitimizing hyper-nationalist holocaust-deniers like Abe for geopolitical reasons want you to think, but in reality there's still a lot of fuckery going on, to use the technical term.
Can you point to something I can read on this? I don't want this to sound like "I doubt what you're saying", I just know little about Japan after WWII and didn't think full scale genocides were going on. Of course, even after the wars, Japan does not have the best record regarding, uh, being nice to other ethnic groups.
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.