In the notorious Nanjing Massacre during World War II, about 300,000 Chinese people lost their lives and 20,000 women were raped, said J. Fred Arment, executive director of International Cities of Peace in a video speech.
This history makes people remember the war and makes them more aware of the significance of peace, he said.
Nanjing in east China's Jiangsu Province was an ancient capital for six of China's dynasties. The freezing winter of 1937 saw brutal killing by invading Japanese troops. One person was killed every 12 seconds.
A total of 172 cities in about 50 countries and regions are now on the International Cities of Peace list, according to its website, including Coventry in England, Bern in Switzerland, Amsterdam in the Netherlands and Berlin, Germany.
Liu Cheng, UNESCO Chair on Peace Studies and a professor at Nanjing University, said that there were certain requirements for becoming an International City of Peace.
"For instance, the city might be traumatized by war or have witnessed big peace-related events," he said. "It should also be advanced in peace studies and activities."
More than two years ago, Liu's Institute for Peace Studies, together with the Institute of Nanjing Massacre History and International Peace, submitted an application to the International Cities of Peace.
"What was left by history was not hatred, but our awareness of peace," he said.
"Nanjing is among the cities that felt the greatest pain in World War II, and the Nanjing Massacre left us indelible memories," said Zhang Jianjun, executive chairman of the Institute of Nanjing Massacre History and International Peace. "So we understand better how valuable peace is."
Peace education has always been emphasized in Nanjing. Nanjing University is the first university to conduct research on peace studies in China.
Kawasaki Akira, a representative with the Japanese NGO Peace Boat, said that the designation of Nanjing as an International City of Peace could help China cherish peace and Japan reflect on itself.
"Peace does not only mean 'no war,'" said Lee Ji Won, a professor at Daelim University of Republic of Korea. "Peace also means reducing violence, improving justice and safeguarding human rights."
"Peace is the only way out for human beings," said Cao Lubao, a publicity official with Nanjing City. "People from all countries should take lessons from history and enhance communication."