To understand the cause of the Nanking Massacre, you must, to a certain extent, understand the Sino-Japanese War. Korea had been China's most important client country, but with it being opposite the Japanese islands and with its natural resources, the Japanese grew interested. In 1875, the Japanese forced korea to open up to foreign trade and to declare itself independent from China. A war broke out between China and Japan. Although the Chinese had a massive force, the Japanese were more modernized and overpowered the Chinese. After the Japanese claimed victory in Shanghai, they marched on to Nanjing. The Chinese were a threat, and they needed to wipe them out.