Photo taken on Dec. 13, 2018 shows the scene of the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, east China's Jiangsu Province. (Xinhua/Ji Chunpeng)

Photo taken on Dec. 13, 2018 shows pigeons flying during the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, east China's Jiangsu Province. (Xinhua/Ji Chunpeng)

Photo taken on Dec. 13, 2018 shows the scene of the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, east China's Jiangsu Province. (Xinhua/Ji Chunpeng)

Photo taken on Dec. 13, 2018 shows the scene of the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, east China's Jiangsu Province. (Xinhua/Ji Chunpeng)

Photo taken on Dec. 13, 2018 shows pigeons flying during the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, east China's Jiangsu Province. (Xinhua/Ji Chunpeng)

Photo taken on Dec. 13, 2018 shows the scene of the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, capital of east China's Jiangsu Province. (Xinhua/Li Bo)

Photo taken on Dec. 13, 2018 shows the scene of the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, capital of east China's Jiangsu Province. (Xinhua/Li Bo)

Photo taken on Dec. 13, 2018 shows the scene of the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, capital of east China's Jiangsu Province. (Xinhua/Li Bo)

Photo taken on Dec. 13, 2018 shows the scene of the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, capital of east China's Jiangsu Province. (Xinhua/Li Bo)

Photo taken on Dec. 13, 2018 shows the scene of the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, capital of east China's Jiangsu Province. (Xinhua/Li Bo)

Photo taken on Dec. 13, 2018 shows the scene of the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, capital of east China's Jiangsu Province. (Xinhua/Yang Lei)

Photo taken on Dec. 13, 2018 shows the scene of the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, capital of east China's Jiangsu Province. (Xinhua/Yang Lei)

China's national flag flies at half mast during the state memorial ceremony for China's National Memorial Day for Nanjing Massacre Victims at the memorial hall for the massacre victims in Nanjing, capital of east China's Jiangsu Province, Dec. 13, 2018. (Xinhua/Li Bo)