Citing specific evidence from the material about the Rape of Nanking, why did this genocide occur?

Everything you need to know about the assignment will be in this link. Please message me if the link doesn't work for you. 

I don't need the essay to be done. I just need someone to point me to the right directions. 

Sources and general answers for the assignment. 

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Just the general answers and sources (websites)

http://www.ctevans.net/Nvcc/HIS112/Assignments/Genocide.html