What was the significance of the Nanking Massacre in the context of World War II?
The behaviour of Japanese troops to Chinese civilians in 1937 and afterwards did much to increase the isolation of Japan and increased Chinese resolve to resist the invasion.
Starting in July 1937, Japanese soldiers in China were tolerated -- and then encouraged -- to treat the Chinese with complete contempt. Imbued with a belief in national and racial superiority, and subjected to a strict discipline, maltreatment of captured Chinese was one of the few outlets ordinary Japanese soldiers had -- particularly when many of their officers encouraged it.
Japanese soldiers began to murder POWs and commit rapes even in Shanghai, and continued these practices as they advanced on Nanking through the Autumn. With the defeat of the Chinese Nationalist forces screening Nanking, large numbers of POWs and Chinese civilians fell into Japanese hands.
While there might be some validity to the Japanese belief that stray Chinese solders might turn into guerrillas, the systemic mass murder of tens of thousands of POWs and male civilians was often accomplished through great cruelty: Burying captives alive, using them for live bayonetting practice, even an apparent beheading contest. At least some 20,000 women in Nanking were raped -- and very often murdered afterwards.
Reasonable estimates hold that some 200,000 Chinese were murdered in the five weeks after Nanking was captured. 22 courageous Westerners sheltered another 200,000 and bore witness to many atrocities. Many Japanese soldiers were naive enough to photograph their crimes (often with newly looted cameras) and Chinese film developers back in Shanghai ensured many copies of their films found a way into other hands.
If the Japanese were hoping to horrify Chinese into surrender, the Nationalists continued to fight... regardless of the cost. It is likely that 15,000,000 Chinese died in the coming years. Japan's behavior alienated many sympathizers in the West and led to Tokyo's growing isolation. The wartime behaviour of Japan in China still colours relations between the countries, and probably will for years more to come.