Nanjing, which was formally known as Nanking or Nankin, was the capital of China until August of 1937. At that time, Japanese forces had effectively attacked and captured Shanghai and were on a march to attack Nanjing. After losing the battle of Shanghai, Chiang Kai-shek made the decision to take his army into the countryside as opposed to losing much of his force in a hopeless battle to defend the capital.
To understand some of the hostility between China and Japan today, one needs to understand the ramifications of the results of the Second World War, or as it is known in China, the Japanese War of Aggression.
While Germany as a country has profusely apologized and each year shows national contrition for its atrocities committed against the Jewish people, there are many Japanese politicians, including President Shinzo Abe, who refuse any sign of remorse. Some even claim despite overwhelming documentation, that the events documented did not happen.