In re: to the Nanjing massacre, it wasn't a communist government at the time. And there is a lot of contemporary reporting of the issue. The stories of atrocities coming out of China were the motivation for the US to impose sanctions on Japan. So, at the time it was fairly well known what was going on.

Also, the war crimes tribunals set up after the war went through a lot of effort to document what happened. This was in the hands of the allied powers, but was mostly done by the US and the UK b/c of Russia's limited participation in the PTO until the very end of the war.

In regards to Korea, they have a lot of mineral resources that Japan needed.