In 1937, seeking mineral resources, the Japanese invaded China. In December, the Japanese conquered the Chinese capital city of Nanjing. Beginning on December 13, 1937, the massacre lasted six weeks. The perpetrators also committed other war crimes such as mass rape, looting, and arson. The massacre was one of the worst atrocities committed during World War II. Over the first six weeks of the occupation, the Japanese murdered anywhere from 40,000-300,000 Chinese citizens. Beyond that, an estimated 20,000 women were raped, and the number of children and elderly women who were raped has not been accurately counted. The reason for the wildly disparate casualty estimates is that the Japanese destroyed most of the records containing information about this time period, leaving little first-hand information for historians to uncover. There is even a story that was told at the time (and which a judge in 2005 said could not legally be proven false) that a pair of Japanese soldiers held a contest between themselves to see who could kill 100 civilians first, using nothing but a sword.

After the Japanese surrender in late 1945, the Chinese held tribunals to determine the guilt of officers involved in the Nanking Massacre. Only seven soldiers who were tried for the massacre were found guilty of a capital crime, and those seven were executed. Others were found guilty of lesser sentences, but not put to death. Several memorials have been built to commemorate the suffering of the Chinese people during the Nanking Massacre.

The Japanese Army had pushed quickly through China after capturing Shanghai in November 1937. By early December, it was on the outskirts of Nanjing. The speed of the army's advance was likely due to commanders allowing looting and rape along the way. As the Japanese approached, the Chinese army withdrew the bulk of its forces since Nanjing was not a defensible position. The civilian government of Nanjing fled, leaving the city under the de facto control of German citizen John Rabe, who had founded the International Committee for the Nanking Safety Zone. On December 5, Prince Yasuhiko Asaka was installed as Japanese commander in the campaign. Whether Asaka ordered the Rape, or simply stood by as it happened, is disputed, but he took no action to stop the carnage.

The first Japanese troops reached the city on December 13, and faced little resistance. The massacre began the same day, with Japanese troops running entirely unchecked. Chinese soldiers were summarily executed in violation of the laws of war. Women and girls were raped en masse and looting was widespread. Due to multiple factors, death toll estimates vary from 40,000 to over 300,000, with rape cases ranging from 20,000 to over 80,000 cases. However, most credible scholars in Japan, which include a large number of authoritative academics, support the validity of the International Military Tribunal for the Far East and its findings, which estimate at least 200,000 murders and at least 20,000 cases of rape.

The massacre finally wound down in early 1938. John Rabe's Safety Zone was mostly a success, and is credited with saving at least 200,000 lives. After the war, the Japanese commanding officers Iwane Matsui and Isamu Cho were found guilty of war crimes for their actions at Nanjing, and were hanged in 1948. Prince Asaka, as part of the Imperial Family, was granted immunity and never tried. The massacre has remained a wedge issue between modern China and Japan. Historical revisionists and nationalists in Japan have been accused of minimizing or denying the massacre