Here are some past posts on this topic:

• During the second Sino-Japanese war, was the "Rape of Nanking" conducted as a policy direction from the Japanese Officers or had they lost control of their Enlisted men?
  ^(27 Nov 2013 | 32 comments)
  ^(/u/AsiaExpert details the multifaceted analyzes of Nanking Massacre's causes accompanied by a vast list of recommended reading.)

• What was the Japanese strategy behind its invasion & subsequent massacre of Nanjing?
  ^(28 Aug 2014 | 4 comments)
  ^(/u/eighthgear puts the massacre in context with longer term trends Japanese imperialism and militarism while /u/ParkSungJun describes the more proximate causes like the hard fought Battle of Shanghai.)