To understand why or how the Nanking Massacre happened we must first understand the history between China and Japan. Both China and Japan, the two most powerful countries of Eastern Asia, have continually fought each other for territory and power. One of the most important pieces of land fought over was of Korea. Korea had mainly been a tributary state of China for most of history. During the 19th century Japan had grown into a more military power and set its aims on gaining control of Korea, which lead to the First Sino-Japanese War. In 1894 Japanese troops attacked the Chinese troops in Korea setting off the war. Only one year later Japan had defeated China and declared Korea independent from China's rule and demanding China to pay 200 million taels to Japan.
After the First Sino-Japanese War Japan continued to build a strong military and focused a large majority of its time and energy on doing so. While in China the government and its citizens continued to deal with internal struggles to establish a unified government. The Chinese Nationalist party and the Communist party continually fought over government power in China. After World War I Japan wanted to expand its military dominance over Asia and saw China as their next victims. In 1931 Japan began to force its military into northern China. By 1937 Japan had taken control of Shanghai and its next move was to march towards Nanking.
Here you can see the short distance from Shanghai and Nanking |