Short answers, pulled from the story.
The Nanjing Massacre began on the 13th of December 1937 when the gates of Nanjing were breached by the Imperial Japanese Army and continued unchecked through the following weeks into early 1938. The International Committee for the Nanking Safety Zone reported 450 cases of murder and rape to the Japanese embassy by the 5th of February 1938.
Prince Yasuhiko Asaka, a member of the imperial family, issued an order to kill all captives before the city's capture which sanctioned the crimes that followed. General Iwane Matsui commanded the Japanese military forces that carried out the massacre and planned to use mustard gas and incendiary bombs to annihilate the capital.
The International Military Tribunal for the Far East estimated the death toll to be more than 200,000 while newer estimates range from 100,000 to 200,000 and some scholars argue for figures as high as 340,000. Robert O. Wilson provided a conservative estimate of 100,000 people slaughtered in cold blood including thousands of soldiers that had thrown down their arms.
Toshiaki Mukai and Tsuyoshi Noda participated in a killing contest to see who could kill 100 people first using only a sword and both officers surpassed their goal during the heat of battle. Noda killed 105 people while Mukai killed 106 people and journalists decided to begin another contest to kill 150 people.
The Nanking Safety Zone led by John Rabe is credited with saving at least 200,000 lives by providing sanctuary for hundreds of thousands of Chinese refugees. Minnie Vautrin sheltered up to 10,000 women at Ginling Girls College while Bernhard Sindberg and Karl Gunther offered refuge to approximately 6,000 to 10,000 Chinese civilians at a cement factory.
The International Military Tribunal for the Far East estimated that Japanese soldiers committed approximately 20,000 cases of rape in the city during the first month of the occupation. Iris Chang estimated that the number of Chinese women raped by Japanese soldiers ranged from 20,000 to 80,000 and some accounts suggest there were 80,000 rapes in total.