>>45
Not to put too fine a point on it, but there was a large German commercial and political (read: Nazi Party members) presence in Nanking at the time.

The Japanese atrocities they witnessed so horrified Hitler himself that he very nearly ended the alliance with Japan then and there. This little fact was found in the Nazi archives captured at the end of the war.

Not that the Japanese and Germans were especially comfortable allies in the first place. The Japanese had very close trade ties with Poland before the war, and Japanese military intelligence shared intelligence on intercepted Soviet military radio transmissions with Polish military intelligence, who were VERY good at translating and decrypting them, and who were (ironically enough, or maybe not) rather more afraid of Stalin than they were of Hitler. The Japanese were very unhappy indeed on the morning of September 2nd, 1939.

Or, if all that's tl;dr, this is one of those universally accepted historical facts that even the political extremists outside Japan have always accepted, and any attempt to sow doubt about it is right up there with, say, denying that the First World War took place. "How do you know? You weren't there, and all the people who supposedly took part are dead now. YOU CAN'T PROVE NOTHIN!"

Hitler wanted China to be a German client state in the Far East. China, or at least the coastal regions, was full of German arms exporters and German military advisors. Hitler wanted Chiang Kai-Shek as an ally in the worst way. Japan's invasion of China seriously pissed him off.

And Japan was very happy to trade both commerce and intel on the Soviets with Poland. Ironically enough the Japanese military junta was terrified of Stalin, but thought the US was a nonentity that would go back to isolationism if they made a sufficent show of force.