China is facing a critical period in its history: the economy is unstable; its population is aging while the birth rate declines. Its social policies are also under attack. The Chinese government is being accused of harsh treatment of its religious minorities, a fact that it steadfastly denies. While Chinese President Xi Jinping’s father, Xi Zhongxun, lobbied for a law to protect freedom of the press, President Xi Jinping has tightened his grip over the country’s media. He is also curtailing the ability of non-governmental organizations to promote human rights and public health. And the Chinese government is ruthless in confronting its opposition. Given this landscape, a lesson from history is worth remembering.

In May 1966, the Chinese leader Mao Zedong launched a socio-political movement called the Cultural Revolution, formally known as the Great Proletarian Cultural Revolution, which lasted 10 years. Millions of people were persecuted, abused, and killed, and millions more were forcibly displaced from the cities to rural areas and made to work for the benefit of the regime. Over 10 million youth from urban areas were relocated under the Down to the Countryside Movement policy. Schools and universities were closed so that students could dedicate themselves to the “revolutionary struggle.” Students were encouraged to demolish the “Four Olds"—old customs, old habits, old culture, and old thinking-. During the process, old temples, works of art, and buildings were destroyed, particularly in Beijing.

Authority figures in society, including teachers, school administrators, Communist Party members, and officials, were purged, including high-ranking members of the party.

China’s President Xi Jinping, whose own father (a leader of the Communist Revolution) was jailed for 16 years, was also a victim of those policies. Xi Jinping’s father was denounced as a counter-revolutionary, and Xi Jinping’s own mother was forced to accuse him. He was sent to a tiny village in China’s northwest to be “reeducated” by peasants.

Historian Jonathan Spence wrote, “Mao began to fear that the Chinese Communist Party was becoming too bureaucratic and that Party officials and planners were abandoning their commitment to the values of communism and revolution.” As a result, he endorsed the attack on authority figures whom he believed had become too complacent and anti-revolutionary.

The purge carried out by the government affected not only people in the lower ranks but also senior officials, who were accused of taking the “capitalist road.” China’s youth responded to Mao’s call by forming Red Guard groups throughout the country. The death toll between 1966 and 1969 has been estimated from various sources at about 500,000 to two million people (some estimates were even higher).

One of the testimonies of the abuses carried out during one of that country’s darkest times was that of Ms. Song Binbin, daughter of Song Renqiong, one of China’s leaders known as the Eight Immortals. In 2014, Song Binbin publicly repented for her participation in the attacks against a former teacher, Bian Zhongyun, who was at the time deputy headmaster at the school. The attacks culminated in the mob beating her to death.

Appearing at the school affiliated with Beijing Normal University, Song Binbin said, “Please allow me to express my everlasting solicitude and apologies to Principal Bian. I failed to protect the school leaders, and this has been a lifelong source of anguish and remorse.”

Ms. Song Binbin’s testimony did not appease Bian Zhongyun’s widower, who, since his wife’s death, has tried to keep his wife’s memory alive and to obtain an honest apology for the perpetrators of his wife’s assassination. “She [Ms. Song Binbin] is a bad person for what she did,” he declared. And he added, “The entire Communist Party and Mao Zedong are also responsible.”

Milestones that reflect the madness of crowds are exemplified by Ms. Song’s testimony and the testimony of Zhang Hongbing. Zhang was a lawyer who, with his father, denounced his mother, Fang Zhongmou, and made her the target of a brutal killing. At the time of his mother’s death, Mr. Zhang was only 16 years old.

What makes this case particularly painful are the circumstances of Mrs. Fang’s death. “They beat her, bound her, and led her from home. She knelt before the crowds as they denounced her. Then they loaded her on to a truck, drove her to the outskirts of town, and shot her,” described Tania Branigan in The Guardian. “My mother, father, and I were devoured by the Cultural Revolution,” declared Mr. Zhang. And he added, “It was a catastrophe suffered by the Chinese nation.”

Mr. Zhang’s testimony shows the extent to which a person’s mind can be so impacted by fear that they could consider their own mother an enemy. He says that at the time he told his mother, “If you go against our dear Chairman Mao, I will smash your dog’s head.” And he added, “I felt this wasn’t my mother. This wasn’t a person. She suddenly became a monster… She had become a class enemy and opened her bloody mouth.” The last time he saw his mother was when she knelt on a stage hours before her death.

More than four decades after his mother’s death, Mr. Zhang is trying to atone for his unrelenting sense of guilt by publicizing the circumstances of her death and by calling for the preservation of her grave in Anhui province, their hometown.

The cases described are just a few examples of a tragic moment in China’s history. Public discussion of the Cultural Revolution is still relatively limited in China, and news organizations are prevented from mentioning in detail what happened during that time. However, the Chinese government still needs to clearly condemn these events. As Ms. Song Binbin declared, "How a country faces the future depends in large part on how it faces its past."