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.

>did that actually happen or was it communist fabricated propaganda?

It actually happened, and even Nazis say it did. It's not a communist fabrication, nor were most of the people killed communists; the CCP didn't take over China until 1949.

>my second question is that why did Japan invade china and korean?

Japan sought the establishment of the Greater East Asia Co-Prosperity Sphere, a thinly-veiled attempt at creating a larger Japanese empire, subjugating other countries for their resources, and ousting other empires from territory they wanted. Japan sought both superiority over other nations and resources to assist its economy, not least oil, to avoid the USA being able to pressure the country via further embargoes.

Read Iris Changs book "The Rape of Nanking " over 200,000 men where slaughtered over 20,000 rapes by Japanese soldiers.And Japanese invaded because they wanted China and Korea,conquest very simple.

Adding on to the other answers, Japanese soldiers were expecting an easy victory in China, and when Japan started taking far more casualties than expected almost immediately, the Japanese reacted brutally: partly in the hopes of causing terrified acquiesce, and partly because the Japanese soldiers were just that furious.

It might seem odd, but Japanese training actually emphasized brutality as a means to make up logistical shortfalls, with beatings and bizarre punishments common in training as a result. Naturally, civilians under them were treated even worse, and this wasn't helped by Japanese propaganda lying about how "grateful" the locals were to Japan, which made the subsequent local resistance also feel unjust at best, leading to worse reprisals.