As previous posts have indicated, the response to the Nanjing Massacre was largely nonexistent because there wasn't much awareness of its occurrence. One of the great services that social media such as Twitter and Facebook are providing, in connection with texting and real-time communication in general, is that it is becoming harder to isolate and hide such atrocities. Too many may still choose to do nothing, but it's becoming much harder to plead ignorance.
I think the response of the world to this massacre is based on the lack of knowledge that the West had of what was going on in China at this time. This meant that the massacre unfortunately largely went unnoticed and unobserved. This is an excellent example of the ways in which massacres can occur and large percentages of the world's population can be blissfully unaware.
Like the initial persecution of the Jews, the rape of Nanjing was not really known to the outside world. Part of the reason for this was due the time in which this took place. Communication was generally slower on many levels. Moreover, not many nations could do anything to really help. At this time, most of Asia was still poor. The only power was Japan and they were doing the attacking.
Only a handful of western accounts emerged from the event, mostly from missionaries. But they did not make it into Western media coverage, and in reality, it was only after the surrender of Japan that the full extent of the atrocities, many of which were extensively photographed and documented, became known, and as the previous post says, a number of Japanese officers (though notably not their commanding officer, Prince Asaka) were held legally accountable for their crimes.
There was not much international reaction to this atrocity. The atrocity was not very well-known in the West. China was very far away and it was hard to get any information about what was happening there since Japan controlled much of the country. Therefore, there was not much of a reaction until after the war. At that point a number of Japanese officers were prosecuted for the atrocity.