From Wikipedia

The Rape Of Nanking

Part Two

Estimates of the death toll vary widely. Aside from the absence of accurate, comprehensive records of the killings, other contributors to the wide variance in estimates of the death toll include differences in definition of the geographical area, time period and nature of the killings to be counted. The Nanking Massacre can be defined narrowly to count only those killings happening within the Nanking Safety Zone, more broadly to include killings in the immediate environs of Nanking, or even more broadly to include the six counties around Nanjing, known as the Nanjing Special Municipality. Similarly, the time period of the massacre can be limited to the six weeks following the fall of Nanking or it can be defined more broadly to include killings from the time the Japanese Army entered Jiangsu province in mid-November until late March 1938. Variations in estimates based on the nature of the killings revolve around the question of whether the killings of captured Chinese soldiers and suspected guerrillas constituted legitimate executions.

The International Military Tribunal of the Far East estimates 260,000 casualties; China's official estimate is 300,000 casualties, based on the evaluation of the Nanking War Crimes Tribunal. Japanese historians estimate a lower death toll, in the vicinity of 100,000-200,000. Some claim the existence of only 40,000 deaths or even deny that a widespread, systematic massacre occurred at all, claiming that any deaths were either justified militarily, accidental or isolated incidents of unauthorized atrocities. These negationists claim that the characterization of the incident as a large-scale, systematic massacre was fabricated for the purpose of political propaganda.