When Doctors Became Killers: The Story of Unit 731

 

I want to start today with a building.

 

Not a famous building. You won’t find it in architecture books or tourist guides. In fact, for most of the twentieth century, the people who built it did everything in their power to make sure you’d never hear about it at all.

 

This building was located on a flat plain near the city of Harbin in what is now northeastern China. It was the size of a small town. It had laboratories, dormitories, an airfield, and a prison block. It employed at its peak up to five thousand people — many of them some of the finest-trained medical scientists in Japan.

So what was this place?

 

Officially, it was called the Epidemic Prevention and Water Purification Department of the Kwantung Army. Sounds almost noble, doesn’t it? Preventing epidemics. Purifying water.

But it was a lie. A deliberate, bureaucratic lie. Because what happened inside that compound between 1936 and 1945 was not the prevention of disease. It was the deliberate creation of it. On human beings. Alive.

This place was known to the world as Unit 731. And by the time we finish today, I think you will agree with me that it represents one of the most disturbing chapters in the entire history of modern science. 

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 But here’s what makes this story more than just a horror story. It’s a story about how perfectly normal, educated, credentialed human beings — doctors, scientists, researchers — can cross a line so catastrophic that even war itself can barely explain it. It’s a story about ideology, about empire, about the weaponisation of science. And it’s a story about how the world, when it had the chance to deliver justice, looked the other way.

So, let’s go back to the beginning.

 THE WORLD THAT MADE UNIT 731

To understand Unit 731, you have to understand Japan in the late nineteenth century. And to understand Japan, you have to imagine waking up one morning and realising that everything you thought made your civilisation great — your traditions, your social order, your military culture — might not be enough to survive in the new world.

What would you do? How far would you go to protect your nation?

That is essentially the crisis Japan faced in 1853, when an American naval commodore named Matthew Perry sailed four warships into Tokyo Bay. The Japanese called them the kurofune — the black ships. And what Perry said, politely but unmistakably, was: open your ports to trade, or face the consequences. Japan, which had been largely closed to the outside world for over two centuries, suddenly found itself staring down the barrel of Western industrial and military power.

The response was extraordinary. Within a generation, Japan transformed itself. It abolished its feudal system, built a modern army and navy modelled on Germany and Britain respectively, established universities, created railways, sent students abroad to learn everything from surgery to shipbuilding. The slogan of the era was

fukoku kyōhei — ‘enrich the country, strengthen the military.’ Japan was going to modernise, or it was going to be colonised. Those were the only options its leaders could see.

 

And modernise it did. Spectacularly. By 1895, Japan defeated China in war. By 1905, it defeated Russia — the first time in the modern era that an Asian nation had beaten a European power. Think about what that meant. Not just to Japan, but to the entire colonised world. Japan was proof that it could be done.

But Japan had also absorbed something else from watching the Western powers. It had absorbed the logic of empire. If you want to be powerful in the modern world, you don’t just build factories. You build colonies. You seize territory. You extract resources. You dominate other peoples. Japan annexed Korea in 1910. It took chunks of China. And in 1931, a group of rogue army officers in Manchuria staged a false-flag explosion on a railway line they themselves controlled, used it as a pretext to invade, and within months had created a puppet state the size of Western Europe.

 

Manchukuo — Japan’s puppet state in Manchuria — was established in 1932. It provided resources, territory, and crucially, distance from Tokyo’s oversight. The Kwantung Army that controlled it answered, in practice, to no one.

 

It is in that context — a militarised empire, governed by an army that operated with near-total autonomy thousands of miles from civilian authority — that Unit 731 was born.

 THE MAN: SHIŌŌ ISHII

Every atrocity has an architect. And Unit 731’s was a man named Shirō Ishii.

Ishii was born in 1892, the son of a landowner. He studied medicine at Kyoto Imperial University — one of the most prestigious universities in the country. He was, by all accounts, brilliant. He was also flamboyant, hard-drinking, charming, and possessed of a ruthlessness that his colleagues either admired or feared.

What kind of man decides that experimenting on living human beings is a good idea?

Here’s the uncomfortable truth: not a monster. Not a cartoon villain. A man who was, in many ways, the product of exactly the system he served. A man with a medical degree, a doctorate, a career to build, and a government telling him that what he was doing was not a crime — it was a patriotic duty.

In the late 1920s, Ishii toured Europe and the United States, studying developments in bacteriology and military medicine. He came back with a conviction: biological weapons were the future of warfare. Cheap to produce. Potentially devastating. And Japan needed them.

He lobbied the military establishment relentlessly. And eventually — in 1936 — Emperor Hirohito himself signed Imperial Edict No. 659, formally authorising the creation of the unit.

I want you to hold onto that detail. This was not a rogue operation. This was not a few mad scientists acting outside the system. This had imperial authorisation. Government funding. Military logistics. Academic partnerships with the country’s top universities. Unit 731 was not a shadow programme. It was a state programme.

Construction began on the Pingfang compound in 1938. By the time it was fully operational, it covered nearly six square kilometres. It had its own railway spur, its own water supply, its own power station. It had a prison block with capacity for four hundred prisoners at any given time — though the total number processed through the facility, over the course of its operation, runs into the thousands.

 

At its peak, Unit 731 employed approximately 3,000–5,000 staff — including military physicians, bacteriologists, chemists, engineers, and support personnel. Many held elite university degrees. Several had doctorates.

 

And the prisoners? The researchers had a name for them. They called them maruta.

Maruta. It’s a Japanese word. It means ‘logs.’ Wood. Timber. Not people. Not patients. Not even enemies. Logs. To be used. To be processed. To be incinerated when no longer needed.

That word tells you everything you need to know about the ideology that made Unit 731 possible. 

WHAT THEY DID: INSIDE THE COMPOUND

Now I have to tell you what happened inside those walls. I want to do this clearly and without sensationalism, because the facts are more than disturbing enough on their own.

What exactly was Unit 731 researching?

Three broad categories. Biological weapons. Human physiology under extreme conditions. And the field deployment of both.

On the biological weapons side: researchers at Pingfang cultivated and weaponised some of the most dangerous pathogens known to science. Bubonic plague. Cholera. Anthrax. Typhoid. Botulism. They bred millions of infected fleas, placed them in specially designed ceramic bombs, and tested whether they could be dropped from aircraft to trigger epidemics in enemy populations.

To test these weapons, they needed to know how lethal they were. At what dose. By what route of transmission. Over what period of time. And to find out, they used the maruta.

Prisoners were infected by injection. By being forced to eat contaminated food. By being placed in sealed chambers filled with aerosolised pathogens. By being exposed to cages of infected fleas.

The progression of the disease was observed. Recorded. Measured. And if a prisoner survived one infection — sometimes they were reinfected. Sometimes with multiple diseases simultaneously.

But it didn’t stop at biological weapons. There were experiments on frostbite. Manchuria’s winters reached minus forty degrees Celsius. Prisoners were taken outside, their limbs exposed until the tissue froze solid, and then researchers tested which rewarming methods were most effective. Hot water. Open fire. Bodily warmth. The ‘results’ were published after the war — in Japanese medical journals — under the researcher’s real name. With no consequences whatsoever.

There were pressure chamber experiments. Prisoners placed in sealed chambers while the atmospheric pressure was altered to extremes — studying what happens to the human body. There were vivisections — live surgery, without anaesthesia, on conscious human beings.

How is it possible that trained physicians — people who took oaths to do no harm — could do this?

Partly, it was ideology. The prisoners were not, in the mental framework of the unit’s staff, fully human. They were Chinese, Korean, Mongolian, Soviet. They were colonial subjects, prisoners, enemies. And in a culture that had constructed an explicit racial hierarchy with ethnic Japanese at the apex, the moral universe simply did not extend to them.

Partly, it was institutional pressure. In a military hierarchy that valued obedience above all else, refusing a direct order was not just professionally dangerous — it was culturally shameful. The sociologist James Waller, who has studied perpetrator psychology in genocide and mass killing, argues that ordinary people can commit extraordinary atrocities when three things align: an authority structure that rewards compliance, a cultural construction of the victim as less than human, and an institutional routine that normalises the violence. All three were present at Pingfang. Every day.

“We removed their organs while they were still alive. I was very young and did not fully understand what I was doing. But I understood they were alive. That I cannot forget.”

— Yoshio Shinozuka, former Unit 731 researcher, testimony given 1997

Yoshio Shinozuka was a teenager when he was assigned to Unit 731. He later became one of the very few Japanese participants who publicly acknowledged what he had done. Notice what he says: ‘I did not fully understand what I was doing.’ That is not exoneration. But it is a window into how systematic dehumanisation works. When you are told from the moment you join an institution that the people in those cells are logs — not men, not women, not children — the act of cutting them open becomes, by some terrible cognitive alchemy, an act of science rather than murder. 

TAKING THE HORROR OUTSIDE THE WALLS

Up to this point, you might be thinking: this is a story about one facility. One compound. However terrible, however vast, it was in one place.

But what if I told you the experiments didn’t stay inside the compound?

In October 1940, a Japanese aircraft flew over the Chinese city of Quzhou in Zhejiang Province and released a mixture of plague-infected fleas and grain from low altitude. Within weeks, the city was hit by an outbreak of bubonic plague.

This was not a coincidence. Chinese historical records and subsequent epidemiological studies have confirmed the connection between Unit 731’s operational calendar and plague outbreaks in at least eleven Chinese cities and towns between 1940 and 1942. Cholera. Typhoid. Anthrax. All deliberately introduced into civilian water supplies and food sources.

 

Conservative estimates suggest the biological warfare field operations linked to Unit 731 caused thousands of civilian deaths in China. Some Chinese researchers place the death toll from field operations alone in the tens of thousands.

 

And here is an almost darkly ironic footnote to these operations: in 1942, Japanese troops advancing through previously infected areas were themselves struck by plague outbreaks. The biological weapons blew back on their own creators. The unit noted this in their reports. And kept going anyway.

Unit 731 was not the only facility. There were satellite units across occupied China: Unit 1644 in Nanjing, Unit 100 in Changchun, others. The programme was not one man’s obsession. It was a network. An industrial-scale infrastructure of biological warfare, embedded in the apparatus of Japanese imperial occupation. Now let’s talk about who the victims were.

THE VICTIMS: WHO WERE THE MARUTA?

I want to spend a moment on the victims. Because in the history of Unit 731, they are too often reduced to numbers, to statistics, to categories. That is, of course, exactly what their captors wanted — and we should resist it.

The largest group of prisoners was Chinese — civilians, resistance fighters, people arrested by the Japanese military police (the Kempeitai) on suspicion of anti-Japanese activity. Many had no meaningful evidence against them. Some were denounced by neighbours in the atmosphere of fear and collaboration that occupation regimes cultivate. Some were simply in the wrong place.

There were Koreans — colonial subjects who occupied a strange, humiliating position in the Japanese imperial hierarchy. Formally Japanese, but treated as racially inferior, politically suspect, useful for labour and for experimentation. There were Mongolians, Soviet citizens, White Russian émigrés from Harbin.

And here is the detail that Americans in the room may find most disturbing: were any of the victims Western?

Yes. U.S. National Archives records contain accounts from returned American and British prisoners of war who reported witnessing experiments on fellow prisoners at a satellite facility in Mukden. At least some Allied soldiers were among those experimented upon. This is not a comfortable fact for the post-war narrative, as we’ll see in a moment.

What did captivity feel like? We have a limited number of survivor testimonies — limited because the unit had a policy of killing all prisoners before the war ended, to eliminate witnesses. But a few survived. And from their accounts and from the testimonies of former unit members who later spoke out, we can reconstruct something of the reality.

Prisoners were given numbers, not names. Cells measured roughly two metres by two metres. They were fed enough to remain useful. They were not told what would happen to them. They waited.

“I was given a number. I had a name, but no one used it. When they came for us, we did not know where we were going or what would happen. We only knew that those who left the block usually did not come back.”

— Testimony of a Chinese survivor, Harbin War Crimes Museum oral history archive

Those who left the block usually did not come back. That sentence. That silence behind that sentence. That is what the word maruta — ‘log’ — was designed to make invisible. 

THE COLLAPSE — AND THE COVER-UP

By the summer of 1945, the war was lost. Japan’s cities were being firebombed. Hiroshima and Nagasaki had been destroyed in August. And on August 8th, the Soviet Union declared war on Japan and launched a massive invasion of Manchuria.

The Kwantung Army, for all its fearsome reputation, collapsed within days. The Soviet advance was too fast, too powerful. And Shirō Ishii issued his final orders to the unit.

What do you do when you’ve spent nine years committing crimes and the people who might prosecute you are at the gates?

You destroy everything. And that is exactly what they did.

Buildings at Pingfang were demolished with explosives. Documents — operational logs, experimental records, prisoner files — were burned in enormous bonfires. The remaining prisoners — perhaps 150 people who were still alive — were executed. Bodies were exhumed and incinerated. The animals and insects were released or killed.

Ishii gathered his staff for a final address. He reportedly told them: ‘The work we have done is a state secret. If any of you reveals what happened here, you and your family will be executed.’

Then they dispersed. Thousands of people. Back to civilian life. Back to Tokyo, to Osaka, to their homes and families. Back to their medical practices and their university departments. And they said nothing. story doesn’t end there. In fact, the next section is almost as disturbing as the one that preceded it.

 THE DEAL: AMERICA’S DARK BARGAIN

 The Tokyo War Crimes Trials began in 1946. Twenty-eight Japanese leaders were tried. Seven were hanged. The tribunal was modelled on Nuremberg, the landmark German war crimes trials that had just concluded in Europe.

So why wasn’t Unit 731 prosecuted at Tokyo?

Because the United States decided not to.

Let me tell you how that decision was made.

In the autumn of 1945, American military investigators made contact with Shirō Ishii and other Unit 731 researchers. What they found astonished them. The American biological warfare programme at Fort Detrick in Maryland — despite enormous wartime investment — had not conducted human experiments. The data that Unit 731 had generated across thousands of subjects, on the lethality of dozens of pathogens under controlled experimental conditions, represented a research windfall that would have taken the American programme years to replicate.

And it was 1947. The Cold War was beginning. The Soviet Union was already a potential adversary. Biological weapons research was strategically critical. And here were the world’s foremost experts in exactly that field, offering their data in exchange for immunity from prosecution.

 

DECLASSIFIED DOCUMENT — 1947  MacArthur HQ memorandum to the U.S. Joint Chiefs of Staff: “The value to the U.S. of Japanese BW data is of such importance to national security as to far outweigh the value of prosecuting Ishii and his colleagues as war criminals.” (U.S. National Archives, Record Group 331. Declassified 1993.)

 

Read that again. They knew. They knew what had been done. They had reviewed the evidence. They had heard the testimony. They knew the scale of it.

And they chose operational advantage over justice.

The biological warfare data was transferred to Fort Detrick. Ishii’s researchers were granted immunity. The prosecution at Tokyo was deliberately shaped to exclude biological warfare. Unit 731 was not mentioned.

What happened to Shirō Ishii after all of this?

He returned to Tokyo. He lived openly. He was interviewed multiple times by American military intelligence and gave detailed accounts of the unit’s activities in exchange for his continued protection. He died of throat cancer in 1959 — in his own bed, in his own home, surrounded by his family.

Several of his colleagues went on to prominent careers. Masaji Kitano — who had succeeded Ishii as the unit’s commander — became a senior executive at the Green Cross Corporation, one of Japan’s largest blood products companies. Others held university chairs. Published academic papers. Received public honours.

“The United States made a Faustian bargain. We knew what these men had done. We knew the scale of it. And we decided that their knowledge was worth more to us than justice was to their victims.”

— Sheldon Harris, Factories of Death (1994)

A Faustian bargain. That’s the precise phrase. And like all Faustian bargains, the bill eventually came due. 

MEMORY, SILENCE, AND THE LONG RECKONING

For most of the post-war period, Unit 731 was simply absent from Japanese public life. Not discussed. Not taught. Not acknowledged.

How does a society forget something of this magnitude?

The same way individuals suppress traumatic memory: through a combination of shame, fear, and the active construction of an alternative story. Japan’s post-war identity was built on two foundations: the atomic bombings, which positioned Japan as victim rather than perpetrator, and the ‘economic miracle’ of rapid reconstruction, which positioned Japan as a success story, a model of peace and development. There was simply no place in that narrative for Unit 731.

The first significant public reckoning came not from the government but from a journalist. In 1981, Seiichi Morimura published a book called Akuma no Hoshoku — The Devil’s Gluttony — based on the accounts of former unit members who had finally broken their silence. It was controversial. Some accused Morimura of exaggeration. But it irreversibly changed the public conversation in Japan.

Today, near Harbin, there is a museum. The Unit 731 Crime Evidence Museum, opened in 1985 and substantially expanded in 2015, stands on the ruins of the Pingfang compound. Preserved laboratory buildings. Artefacts. Documents. Victim memorials. It receives millions of visitors. In China, Unit 731 is not forgotten. It is taught. It is commemorated. It is raw.

 

In 1997, Chinese survivors and victims’ families filed a civil lawsuit in Tokyo against the Japanese government. In 2005, Japan’s courts ruled that while the facts of Unit 731 were “indisputably established,” the plaintiffs had no legal standing to pursue individual compensation. The crimes were acknowledged. Justice was denied.

 

The contrast with Germany is striking. Germany has pursued a sustained programme of legal accountability, educational acknowledgment, and memorial culture that is widely cited as a model of how a society can reckon with its worst crimes. Japan’s engagement with its wartime record has been considerably more incomplete — and this incompleteness continues to strain its relationships with China and Korea to this day. 

WHAT DOES IT MEAN? THE BIG QUESTIONS

I want to step back from the historical detail now and ask a few bigger questions. Because history without reflection is just a list of events. And this story demands reflection.

How do ordinary people commit extraordinary atrocities?

The researcher James Waller argues it comes down to three converging forces: a cultural logic that strips humanity from the target group, an authority structure that normalises and rewards participation, and the incremental nature of escalation — each step is only a little worse than the last. You don’t go from medical school to live vivisection in one jump. You go step by step, each step normalised by institutional routine, peer behaviour, and the language of science.

Hannah Arendt, writing about the Holocaust, coined the phrase the ‘banality of evil.’ She meant that the most extreme human cruelty is not always the product of extraordinary, demonic minds. More often, it is the product of ordinary bureaucratic compliance. Of people doing their jobs. Of career incentives. Of paperwork. Unit 731 was, among other things, a bureaucracy. And bureaucracies are very, very good at distributing moral responsibility until no single person feels fully responsible for anything.

What about the data? If atrocity produced knowledge, should we use it?

This is one of the most genuinely difficult ethical questions the Unit 731 story raises. The frostbite data generated at Pingfang was referenced in military medical training manuals. Some of the pathogen lethality data informed post-war biological weapons research — on both sides of the Cold War. The question of whether that knowledge should ever be cited, used, or built upon in contemporary medical science is not settled.

Medical ethicist Arthur Caplan at New York University argues that using atrocity-derived data provides retrospective legitimation of the methods used to obtain it — that it effectively says: ‘the crimes were worth it.’ Others argue that suppressing the data punishes present-day medicine rather than the perpetrators. Neither position is without cost. What both positions agree on is this: the origins of the data must be acknowledged, not erased. The least we owe the victims is that we do not quietly absorb their suffering into our scientific inheritance without naming where it came from.

And finally: could something like this happen again?

The honest answer is: the conditions that produced Unit 731 are not unique to 1930s Japan. Racial dehumanisation. Military institutions insulated from civilian oversight. The weaponisation of science in the service of national interest. Strategic calculations that weigh operational advantage against accountability and find accountability wanting.

These are not historical conditions. They are perennial conditions. They appear and reappear in different forms, in different contexts, across time. The specific confluence that produced Unit 731 — the Japanese imperial state, the Kwantung Army’s autonomy, Ishii’s ambition, the Cold War’s strategic calculus — will not recur exactly. But its constituent elements are always with us.

The only protection against them is what we are doing right now. Naming them. Understanding them. Refusing to look away.

 THE RETURN TO THE BUILDING

I started today with a building.

That building no longer stands in its original form. The explosions of August 1945 saw to that. But the ruins are still there. The museum is still there. And on the walls, there are photographs. Numbers. The administrative records of the people who were processed through those cells. Each one a human being. Each one with a name — though the unit never used it.

The Japanese word for that building’s purpose was maruta. Logs.

But they were not logs. They were people. People who were afraid. People who had families. People who could not have imagined, when they were arrested, that the modern world — the world of universities and laboratories and peer-reviewed journals and military honour — could produce the thing that was waiting for them in that compound.

The philosopher George Santayana wrote that ‘those who cannot remember the past are condemned to repeat it.’ It is a sentence so often quoted that it has lost some of its power. Let me offer a sharper version, more specific to today’s subject:

“Those who refuse to fully acknowledge the worst of the past do not merely risk repeating it. They actively create the conditions in which repetition becomes possible. Incompleteness is not neutrality. It is a choice. And it has consequences.”

— Adapted from the scholarship of transitional justice

The victims of Unit 731 have been waiting nearly eighty years for a full reckoning. The Japanese government has not issued one. The United States government has not issued one. The researchers were never tried.

What we can do — what today is about — is at the very minimum this: we can remember. We can say their names, where we know them. We can refuse the comfortable forgetting that power always prefers. We can insist that the language of science and the language of national security are never, on their own, sufficient justification for the destruction of human beings.