南京保卫战
The duration of the incident is naturally defined by its geography: the earlier the Japanese entered the area, the longer the duration. The Battle of Nanking ended on December 13, when the divisions of the Japanese Army entered the walled city of Nanking. The Tokyo War Crime Tribunal defined the period of the massacre to the ensuing six weeks. More conservative estimates say the massacre started on December 14, when the troops entered the Safety Zone, and that it lasted for six weeks.