Although there can be debate over what does and does not constitute an act of genocide, the Rape of Nanking is generally not seen as an act of genocide.  Genocide typically refers to efforts to destroy an ethnic group because of their ethnicity.  The Rape of Nanking is not typically seen in this way.  Instead, it is typically seen as a war crime.  It is seen as an incident in which soldiers abused civilians in horrific ways largely because the civilians were in their power, not because of their ethnicity.  The Rape of Nanking was a horrible atrocity, but it is not typically defined as an act of genocide.