Hata's major contribution to Nanking Massacre studies is his book "Nankin jiken" ("The Nanking Incident"), published in 1986, which is a detailed study of the event based on Japanese, Chinese, and English sources that was later noted by historians such as Daqing Yang to be one of the few impartial works of scholarship written on the massacre during the period. The book is known for its relatively low estimate of the death toll, which Hata put at up to 40,000 because he based the number of civilian killing on the work of Lewis S. C. Smythe who conducted a survey of the massacre in the immediate aftermath (War Damage in Nanking Area, Dec.1937 to March 1938, Urban and Rural Surveys) and also exclude Chinese soldiers. Hata's book is acknowledged as the first to discuss what might have caused the massacre, whereas previous books had focused only on the event itself. Hata argued that the Japanese Army's lack of military police and facilities to detain POWs, its ignorance of international laws, and the Chinese General Tang Shengzhi's decision to flee the city without formally surrendering, which left large number of plain-clothes soldiers within the civilian population which was followed by excessive mopping-up operations by the Japanese, among the factors which led to the slaughter.