Photo: screenshot of the CNA documentary Inside Unit 731: Japan's Secret Human Experiments

A documentary revisiting the Imperial Japanese Army's lethal human experiments and biological warfare program has attracted more than 1 million views online, bringing renewed global attention to one of World War II's darkest chapters and a history still little known outside Asia.

Produced by Singapore-based Channel NewsAsia (CNA), Inside Unit 731: Japan's Secret Human Experiments features historians and descendants of survivors from countries including the UK, the US, Japan and Singapore. Through their accounts, the documentary details the germ warfare, human experimentation and other crimes against humanity committed by Unit 731 - officially known as the Epidemic Prevention and Water Purification Department of the Kwantung Army - during Japan's war of aggression.

Several US, Singaporean and Japanese scholars featured in the documentary recently told the Global Times that the evidence of Unit 731's crimes against humanity is irrefutable. The atrocities represent a shared trauma not only for Asian countries subjected to Japanese fascist aggression, but also for all humanity. "More than 80 years after the war, reviving the collective memory of affected countries is essential to safeguarding peace," they said.

Victims of germ warfare

For much of his life, Katsutoshi Takegami knew only that his father had once worked in China. It was not until years after his father's death that a discovery in the family storehouse prompted the 78-year-old resident of Komagane, Nagano Prefecture, Japan, to investigate what that work had involved.

Several thick photo albums had been packed into boxes marked "Nara Army Hospital." Across the black-and-white photographs were references to the "Epidemic Prevention Section" and the "Central China Epidemic Prevention and Water Purification Department."

"My father worked at a local public health center after World War II. He had mentioned that he had worked in China, but never revealed any details," Takegami told the Global Times. "When I saw those photographs, I desperately wanted to know what my father had done in China."

Determined to find an answer, Takegami closed the café he had spent years building, began examining historical records and consulted researchers. He even traveled to China in search of clues.

His investigation eventually revealed that his father had been a member of Unit 1644, also known as Unit Ei 1644, a germ-warfare formation based in Nanjing. The discovery came as a profound shock.

"When people talk about the Imperial Japanese Army's germ warfare, Unit 731 is usually the first thing that comes to mind. In fact, the Japanese military operated many such units," Takegami said.

"The general public knows almost nothing about them, and very few scholars study them. Historical evidence shows that the areas victimized by germ warfare extended far beyond Harbin, where Unit 731 was headquartered. China was by no means the only victimized country," he added.

For Singaporean independent scholar Lim Shao Bin, the realization that this biological warfare network had reached Southeast Asia came on February 14, 2016.

That day, Nobuyoshi Takashima, professor emeritus at the University of the Ryukyus, and his wife took Lim to the former Tampoi Mental Hospital on the outskirts of Johor Bahru, Malaysia. Pointing to a row of buildings, Takashima told him: "This was a wartime production base for Unit 731's plague bombs."

"That cut deeply into me," Lim told the Global Times. "I had studied in Japan since the 1980s and believed I knew a great deal about the crimes committed by the Japanese invaders in China. But I never imagined that Singapore and Malaysia - right on our doorstep - had also suffered at the hands of Unit 731."

From that day forward, Lim began systematically tracing the activities of Japanese germ-warfare units in Southeast Asia.

He explained that the Epidemic Prevention Research Laboratory of the Imperial Japanese Army Medical School established the Epidemic Prevention and Water Purification Department of the Kwantung Army in Pingfang, Harbin, which became known as Unit 731.

Photo: screenshot of the CNA documentary Inside Unit 731: Japan's Secret Human Experiments

Using Unit 731 as the center of its operations, the Japanese military established the North China Expeditionary Army Epidemic Prevention and Water Purification Department in Beijing, known as Unit 1855; Unit Ei 1644 in Nanjing; and Unit Nami 8604 in Guangzhou.

''Yet this network still contained a major missing piece - Southeast Asia,'' Lim said.

The Japanese military established the Epidemic Prevention and Water Purification Department of Southern Expeditionary Army in Singapore under the code name Oka Unit 9420. Together, these interconnected germ-warfare formations constituted what Lim described as the "Unit 731 system."

A map compiled by Lim marks five branches of Oka Unit 9420 across Southeast Asia: the Malaya, Java, Manila, Indochina and Thailand branches. These facilities produced plague bacilli and fleas on a large scale, supplying the Japanese military with biological warfare materials and forming a regional germ-warfare network.

Historical documents indicate that Oka Unit 9420 was the largest Japanese germ-warfare unit operating outside the China theater.

In May 2025, Lim and Wang Xuan, president of the Zhejiang Provincial Historical Society's research association on the Chinese People's War of Resistance Against Japanese Aggression (1931-45), published a book on the Japanese Army's Oka Unit 9420 after five years of extensive research and writing.

The book offered the first comprehensive account of this nearly forgotten unit and was also the first historical monograph devoted to the Southern Expeditionary Army Epidemic Prevention and Water Purification Department.

Work far from over

The CNA documentary traces the origins of Japan's biological warfare ambitions to the 1925 Geneva Protocol, humanity's first major post-World War I attempt to prohibit the use of chemical and biological weapons.

Instead of treating the prohibition as a moral boundary, Shiro Ishii, the notorious commander of Unit 731, recognized the enormous military value of such weapons and regarded them as "a decisive means of winning future wars," according to the documentary.

Unit 731 presented itself publicly as an epidemic-prevention and public health organization. Its real purpose, however, was to conduct secret human experiments in support of biological warfare.

The experiments included infecting victims with plague-bearing fleas, injecting them with dysentery pathogens and subjecting them to frostbite tests in temperatures dozens of degrees below zero.

Japanese soldiers referred to prisoners of war and civilian victims as maruta, meaning "logs," stripping them of their humanity before using them as experimental subjects. Everyone sent into the laboratories eventually died.

As Lim's research shows, the system supporting these experiments extended far beyond Unit 731's headquarters in Harbin.

In February 2018, Lim obtained exclusive access to a reel of wartime film footage shot after 1942 by Tomosada Masuda, a close associate and former schoolmate of Ishii who later became a core member of Unit 731. The footage documented the activities of Oka Unit 9420 in Southeast Asia.

"Harbin's severe winters made it impossible to breed plague-infected fleas on a large scale throughout the year, so Unit 731 urgently needed a warm and humid cultivation base," Lim said. "The optimal conditions for breeding the fleas were temperatures of 27 to 30 C and humidity of around 90 percent. Singapore's climate met all of these requirements."

Another document preserved in a Japanese archive showed that Japanese military "researchers" had successfully isolated a new virus from rats in Singapore and planned to transport it to Unit 731's headquarters in Harbin.

Lim said the virus was not originally native to Singapore but had been deliberately cultivated and introduced by the Japanese military. The Japanese Imperial Army recorded the body temperatures, symptoms and other experimental data of 70 test subjects in detail.

"The documentary examines in depth the crucial role Singapore played in the broader biological warfare operation," Alexis Dudden, a professor of history at the University of Connecticut, told the Global Times. "This not only expands the geographical scope of research into Unit 731's criminal activities, but also makes one point clear: This history cannot simply be characterized as a dispute between China and Japan. At its core, it was a transnational crime done by Japanese imperial expansion."

Dudden also pointed to the testimony of Hideo Shimizu, a former teenage recruit of Unit 731 who appeared in the documentary. Accounts by those who directly witnessed the unit's activities demonstrate that "the work of exposing this dark history is far from over," she said.

The documentary also highlights the stark disparity in postwar accountability.

After World War II, the victorious Allied powers conducted international trials of leading Nazi war criminals and criminal organizations in Nuremberg, Germany. Yet many of Unit 731's "executioners" returned to Japanese society, where they resumed ordinary careers in medicine and other professions.

"In a sense, this forces all of us to ask: How could a person commit such horrifying crimes, then return home and continue to act as a caring partner, a kind friend or a responsible parent?" Dudden said.

Understand history, learn from it

As of press time, "Inside Unit 731: Japan's Secret Human Experiments" has attracted more than 1.13 million views on YouTube and other platforms, with the figure rising.

Its reception raises a broader question: Why has a history rooted in wartime Asia resonated so strongly with audiences across national borders?

"Many people outside China know little or nothing about this history," Dudden said. "For a long time, international understanding of Unit 731 has not merely contained blind spots. It has been marked by widespread historical ignorance - people simply do not know this history."

Dudden cited an example from the university where she teaches. An accomplished professor who held a doctorate from Kyoto University and had spent years living and working in Japan was stunned after watching the documentary.

"I grew up in Bosnia and worked and lived in Japan for many years. My own grandparents died in Nazi concentration camps, yet I had never heard of this history," the professor said.

The professor urged Dudden to introduce the documentary in her classes and help disseminate the historical truth about Unit 731 because "everyone can and must understand this history and learn from it."

At the same time, Dudden emphasized that "understanding history and learning historical knowledge does not mean hating Japan or the Japanese people."

Asked by the Global Times what lessons remembering and reflecting on history can offer for preventing past tragedies from recurring and safeguarding peace and justice, amid Japan's accelerating rightward shift and mounting conflicts worldwide, Dudden said that "choosing silence in the face of atrocities is, to some extent, tantamount to abetting evil."

"Collective numbness and inaction can all too easily give rise to further harm. If humanity is unwilling to reflect together and defend our shared human values, similar tragedies will very likely happen again. Therein lies the answer," Dudden said. 

For historians, years of work with archival materials are essential to identifying decisive evidence and exposing the falsehoods of historical denialists, Dudden said.

"But the challenge remains: Distorted and sanitized narratives have already hardened into mainstream opinion in some countries," she noted. "From this perspective, the historical truth about Unit 731, including that presented in CNA's documentary, deserves to be disseminated widely."

Da Zhigang, a research fellow at the Institute of Northeast Asian Studies of the Heilongjiang Academy of Social Sciences, told the Global Times that the Imperial Japanese Army's germ warfare, chemical warfare and other atrocities were not confined to China.

They also spread across Southeast Asia and the Pacific theater, inflicting profound suffering on people in numerous countries, he said.

Da said the Japanese government has long pursued historical revisionism by deliberately confining its wartime crimes to a bilateral China-Japan framework. It has even substituted concepts and reversed right and wrong, portraying a deliberately launched war of aggression as "an act of self-defense under a complex international environment," he said.

Such narratives seek to reduce the perceived geographical scope of Japan's atrocities and dilute its responsibility for wartime crimes, Da noted.

"Japan's historical revisionism is, in essence, a flagrant violation of the spirit of the UN Charter, international human rights norms and the principles of Japan's own Constitution," Da said. 

"The current Sanae Takaichi administration uses AI and other means to manipulate public opinion, continually poisoning the Japanese public's understanding of history and fundamentally distorting the purpose of peace education," Da said. 

"This erroneous view of history is highly transmissible and has serious spillover effects. It undermines the foundations of Japan's relations with regional countries and risks triggering a chain of negative consequences with lasting repercussions," the expert warned.

Many Asia-Pacific countries share memories of suffering under Japanese militarism as well as a strong emotional resonance rooted in that history. This provides a solid foundation for building a multilateral system of historical accountability, Da said.

Victimized countries across the region should preserve their shared historical memory, work together to expose false narratives promoted by Japan's right wing and fully restore the historical record of Japanese aggression, he added. Such efforts would provide a firm historical basis for upholding regional peace and justice.

For Lim, turning that goal into reality requires overcoming serious obstacles. The countries of Southeast Asia were ruled by different colonial powers during the war, and their archives were consequently preserved in different languages. Records relating to Singapore and Malaysia were largely written in English under British colonial rule, those in Indonesia in Dutch, and those in Vietnam in French.

Already scarce documents are therefore scattered across different countries and divided by linguistic barriers, leaving researchers with very limited material, Lim added.

"Unit 731's germ warfare, human experimentation and other crimes are not only a source of pain for China. They are a shared trauma for all Asian countries that suffered Japanese aggression, and for humanity as a whole," Lim said. "Researching this history is therefore not the responsibility of any one country, but a common responsibility of the international community."

Lim called on countries to strengthen archival sharing and academic cooperation, break down linguistic and national barriers, and jointly uncover more historical truths that have remained buried.

"More than 80 years after the war, reviving the collective memory of affected countries is essential to safeguarding peace," Lim said. "Only by facing history squarely and reflecting on it can we prevent such tragedy from recurring and build a solid foundation for peace."