Following the Mukden Incident in 1931, Japan began its invasion of Manchuria. Because the Communists and the Kuomintang (KMT) were engaged in the Chinese Civil War they were distracted from the reality of Japanese advances. However, in 1937, following the Xi'an Incident, the Chinese communists and nationalists agreed to form a united front. The KMT then formally started an all-out defense against the Japanese threat.

However, the Chinese army was poorly trained and equipped: some regiments were armed primarily with swords and hand grenades and few had anti-tank weaponry. Despite their difficulties, it is likely that China fielded the largest army in the world at the time in terms of troop numbers. Following the Battle at Marco Polo Bridge, which formally started the Sino-Japanese War, the Japanese were swift in capturing major Chinese cities in the northeast.

The incident is considered as the most infamous event in the Japanese invasion of China. In August of 1937, the Japanese army faced strong resistance and suffered heavy casualties in the Battle of Shanghai, effectively destroying the possibility of conquering China in three months. The Battle in Shanghai was bloody as both sides faced attrition in urban hand-to-hand combat. Many historians today believe that the traumatic situation in Shanghai nurtured some of the psychological conditions for Japanese soldiers to march on Nanjing later on.

By mid-November the Japanese had captured the city with help of naval bombardment. The General Staff Headquarters in Tokyo decided not to expand the war due to heavy casualties incurred and the low morale of the troops. However, on December 1, headquarters ordered the Central China Area Army and the 10th Army to capture Nanjing, then the capital of the Republic of China. The Japanese army contained many army reserves who had families back home and expected to return home once the campaign in Shanghai was over. This combined with the trauma of from the Battle of Shanghai nurtured strong sentiments of hate towards the Chinese.

After losing the Battle of Shanghai, Chiang Kai-shek knew the fall of Nanjing would be simply a matter of time. Leaving Gen. Tang Shengzhi in charge of the city for the Battle of Nanjing, Chiang and many of his advisors flew to Chongqing, China's wartime capital for the next seven years. Although walls of defence were constructed, they were never effectively used. In the absence of an organized retreat, the situation before the Japanese entered Nanjing was chaotic. On December 13, the Japanese entered a city virtually free of any military resistance.