Unit 731

• 1. Introduction

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  Unit 731 sat in the Pingfang district of Harbin, disguised as a lumber mill. Local authorities were told the sprawling complex produced timber. The prisoners inside were called "logs" by the staff. An estimated 14,000 people were killed inside the facility itself, and the biological weapons it developed are believed to have caused between 200,000 and 500,000 additional deaths across Chinese cities and villages. This is the story of a secret research division of the Imperial Japanese Army that operated between 1933 and 1945, led by Lieutenant General Shiro Ishii, and whose crimes were concealed for decades with the direct assistance of the United States government. How did a military unit manage to conduct mass human experimentation on this scale? What happened to those responsible? And why did it take until 2002 for any court to formally acknowledge what had happened?

• 2. Building the Machine

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  Shiro Ishii returned from a two-year tour of American and European research institutions with a specific ambition: to develop biological weapons that used humans as their intended victims. His key supporter inside the army was Colonel Chikahiko Koizumi, who had been impressed by the German use of chlorine gas at the Second Battle of Ypres during World War I, an attack that caused 6,000 deaths and 15,000 wounded among Allied forces. The logic behind Japan's biological weapons program drew directly from the 1925 Geneva Protocol, which banned biological weapons in interstate conflicts. Japanese military planners reasoned that the very fact of the prohibition implied these weapons were effective.

  In 1932, Ishii established the Epidemic Prevention Research Laboratory at the Japanese Army Military Medical School in Tokyo. Japan's occupation of Manchuria beginning in 1931 gave the program a crucial resource: unlimited human test subjects. Japanese military planners viewed the Chinese population as no-cost assets and saw a ready supply of test subjects as a competitive advantage in biological warfare.

  The unit Ishii first organized for chemical and biological experimentation in Manchuria was called the Togo Unit, and it operated initially out of the Zhongma Fortress, a prison camp in the village of Beiyinhe, roughly 100 km south of Harbin. Prisoners there were deliberately infected with plague bacteria and subjected to vivisection. A prison break in the autumn of 1934 and an explosion in 1935, believed to be sabotage, led Ishii to close the Zhongma Fortress and seek authorization to relocate to Pingfang, approximately 24 km south of Harbin, to build a much larger facility.

  In 1936, Emperor Hirohito issued a decree authorizing the unit's expansion and its formal integration into the Kwantung Army. By the time the network reached its peak in 1939, Ishii's operation oversaw more than 10,000 personnel across branches in Beijing, Nanjing, Guangzhou, Singapore, and elsewhere. The complex at Pingfang alone covered 6 square kilometers and comprised more than 150 buildings.

• 3. The Experiments

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  Hideo Shimizu joined Unit 731 at age 14, brought in on the encouragement of a former teacher as part of the fourth group of minors assigned to the unit. He recalled being taken to a specimen room where jars, some reaching the height of an adult, held human body parts preserved in formalin, including heads and hands, and the body of a pregnant woman with a fetus visible.

  Prisoners arrived at night in black motor vehicles with ventilation holes but no windows. They were transported through a secret tunnel beneath the central building to the inner prisons, where technicians took blood and stool samples and measured kidney function. Those deemed fit for experimentation lost their names and received a three-digit number, which they kept until death.

  Vivisection was performed on thousands of men, women, children, and infants, often without anesthesia and usually lethal. Former unit member Okawa Fukumatsu admitted in a video interview to having vivisected a pregnant woman. Prisoners had limbs amputated to study blood loss; limbs were sometimes reattached to the opposite side of the body. Some prisoners had their stomachs surgically removed with the esophagus reattached directly to the intestines. Imperial Japanese Army surgeon Ken Yuasa later estimated that at least 1,000 Japanese personnel were involved in vivisection in mainland China, and stated that when he performed such procedures, they were "all for practice rather than for research" and were "routine" among Japanese doctors stationed in the region.

  Nakagawa Yonezo, professor emeritus at Osaka University, watched footage of experiments and executions from Unit 731 while studying at Kyoto University during the war. He later testified about the nature of what he saw: "Some of the experiments had nothing to do with advancing the capability of germ warfare, or of medicine. There is such a thing as professional curiosity: 'What would happen if we did such and such?' What medical purpose was served by performing and studying beheadings? None at all. That was just playing around."

  Army Engineer Hisato Yoshimura led frostbite experiments, taking prisoners outside and submerging their limbs in water at varying temperatures until the limbs froze solid, then striking the frozen flesh with a stick that emitted, in his own documentation, "a sound resembling that which a board gives when it is struck." Unit member Naoji Uezono described in a 1980s interview watching Yoshimura place two naked men in a space 40-50 degrees below zero while researchers filmed until the subjects died, the subjects "suffering such agony they were digging their nails into each other's flesh." In 1950, Yoshimura published an article in the Japanese Journal of Physiology openly admitting to using 20 children and a three-day-old infant in experiments exposing them to zero-degree ice and salt water. When contacted by a reporter from the Mainichi Shimbun, he denied any guilt.

  Researchers at Unit 731 published some of their findings in peer-reviewed journals, describing the subjects not as human prisoners but as nonhuman primates called "Manchurian monkeys" or "long-tailed monkeys." The actual research data, including clinical records of daily pathogen progression within prisoners' bodies, was later held in part by the Library of Congress in three declassified documents, each more than 100 pages long, translated from Japanese into English.

• 4. Weapons on Chinese Cities

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  By 1939, Ishii had condensed his laboratory findings into six pathogens he considered potent enough to ignite large-scale epidemics and stable enough for aerial dispersal: anthrax, typhoid, paratyphoid, glanders, dysentery, and plague-infected human fleas. His delivery systems included biodegradable bombs housing live rats and fleas infected with disease, designed to explode mid-air so the infected creatures descended safely to the ground. He also deployed birds and bird feathers contaminated with anthrax from low-flying aircraft.

  Plague-infected fleas, bred in Unit 731's laboratories, were dropped over the coastal city of Ningbo and over Changde in Hunan Province in 1940 and 1941. These attacks triggered bubonic plague epidemics that killed tens of thousands. A separate expedition to Nanjing involved spreading typhoid and paratyphoid germs into the city's wells, marshes, and houses, and infusing them into snacks distributed to locals. Epidemics followed. Researchers concluded that paratyphoid fever was "the most effective" of the pathogens.

  The facility's production capabilities were formidable. Japanese soldiers testified that the program could manufacture 300 kg of plague bacteria per month, 500-700 kg of anthrax, 800-900 kg of typhoid, and 1,000 kg of cholera. The complex maintained approximately 4,500 flea incubators, each capable of producing at least 45 kg of fleas per cycle. At least 12 large-scale bioweapon field trials were carried out against at least 11 Chinese cities.

  An attack on Changde in 1941 resulted in roughly 10,000 biological casualties. The attack also killed approximately 1,700 Japanese troops in the area, most from cholera, demonstrating the hazards the unit's own weapons posed to Japanese forces.

  Franklin D. Roosevelt issued a public warning in 1943 condemning the attacks after Chiang Kai-shek sent a delegation of army and foreign medical personnel in November 1941 to document evidence from the Changde plague bombing. In December 1944, the Japanese Navy developed plans for Operation PX, also known as Operation Cherry Blossoms at Night, which envisioned Seiran aircraft launched from Sentoku submarine carriers spreading weaponized bubonic plague, cholera, typhus, and dengue fever across San Diego, Los Angeles, and San Francisco. Planning was finalized on the 26th of March 1945, but Chief of General Staff Yoshijiro Umezu blocked it, warning: "If bacteriological warfare is conducted, it will grow from the dimension of war between Japan and America to an endless battle of humanity against bacteria."

• 5. Concealment and Immunity

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  As the Soviet Red Army advanced in August 1945, Ishii ordered the complete destruction of all incriminating materials at Pingfang. The roughly 300 remaining prisoners were killed: one quarter were coerced into hanging themselves with rope provided for the purpose; the remaining three quarters were made to drink potassium cyanide or were killed by injection. The 600 Chinese and Manchurian laborers at the compound were shot. Ishii ordered every member of the group to disappear and "take the secret to the grave."

  Lieutenant Colonel Murray Sanders, a microbiologist and member of America's military center for biological weapons, arrived in Yokohama in September 1945 aboard the American ship Sturgess with no prior knowledge of Unit 731. He received little cooperation until he threatened to bring the Soviets into the investigation. The following morning, a manuscript detailing Japan's biological warfare activities was delivered to him. Sanders took this to General Douglas MacArthur, who secretly granted immunity to Unit 731's physicians, including Ishii himself, in exchange for exclusive American access to their research data on biological warfare and human experimentation.

  The Tokyo War Crimes Tribunal heard only a single reference to Japanese experiments on Chinese civilians, raised in August 1946 by Joseph R. Massey, assistant to the Chinese prosecutor. The claim was dismissed by tribunal president Sir William Webb for lack of evidence. Massey's reference is believed to have been accidental; he was likely unaware of Unit 731's actual operations. Decades later, one of the last surviving tribunal members, Judge Roling, described it as "a bitter experience" to learn that evidence of centrally ordered Japanese war criminality "of the most disgusting kind was kept secret from the court by the U.S. government."

  Historian Sheldon H. Harris later concluded that the Japanese data failed to meet American scientific standards, and that Murray Sanders himself characterized the experiments as "crude" and "ineffective." Harris argued that U.S. scientists primarily wanted the data because of the allure of research that ethical and legal constraints had prevented them from conducting themselves. The contrast with how German perpetrators were treated was stark: while German physicians were brought to trial and their crimes publicized, the United States concealed information about the Japanese program and secured immunity for its leaders. Harris argued that racism informed this double standard.

  Meanwhile, the Truman administration paid stipends to former Unit 731 personnel. From 1948 to 1958, less than five percent of the unit's documents were transferred to microfilm and stored at the U.S. National Archives before being shipped back to Japan.

• 6. Soviet Trials and Postwar Reckoning

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  The Soviet Union, publicly silent on the subject at the Tokyo Trials, pursued its own prosecution. In December 1949, twelve former members of Unit 731 and its affiliated units were tried at Khabarovsk. Among the accused was General Otozuo Yamada, commander-in-chief of the Kwantung Army. The lead prosecutor was Lev Smirnov, who had been one of the top Soviet prosecutors at the Nuremberg Trials. Sentences ranged from 2 to 25 years in Siberian labor camps. All defendants had returned to Japan by 1956.

  The United States dismissed the Khabarovsk proceedings as communist propaganda. However, evidence later emerged suggesting that former Unit 731 members had also passed biological experimentation data to the Soviet government in exchange for their lenient sentences. The Soviet Union subsequently built a biological weapons facility in Sverdlovsk using documentation captured from Unit 731 in Manchuria.

  In Japan, public discussion of Unit 731 began in the 1950s after the American occupation ended. In 1950, former unit member Masaji Kitano co-founded the blood bank and pharmaceutical company Green Cross; the company became the subject of a major scandal in the 1980s after up to 3,000 Japanese contracted HIV through the distribution of blood products the Pharmaceuticals and Medical Devices Agency had deemed unsafe. In 1981, author Seiichi Morimura published The Devil's Gluttony, claiming to reveal the unit's operations, though the book drew controversy for attributing unrelated photographs to the unit.

  In 1983, Japan's Ministry of Education asked historian Saburo Ienaga to remove a reference from one of his textbooks stating that Unit 731 conducted experiments on thousands of Chinese. In 1997, the Supreme Court of Japan ruled that requiring its removal was an illegal violation of freedom of speech. That same year, international lawyer Konen Tsuchiya filed a class action suit against the Japanese government demanding reparations; all levels of the Japanese court system found the suit baseless.

  In August 2002, the Tokyo District Court formally acknowledged for the first time that Japan had conducted biological warfare in China. Presiding judge Koji Iwata ruled that Unit 731, under orders from Imperial Japanese Army headquarters, had used bacteriological weapons against Chinese civilians between 1940 and 1942, spreading plague and typhoid in the cities of Quzhou, Ningbo, and Changde. Victims' compensation claims were rejected on the grounds that they had been settled by international peace treaties.

  In April 2018, the National Archives of Japan released a near-complete list of 3,607 personnel affiliated with Unit 731, made available to Professor Katsuo Nishiyama of the Shiga University of Medical Science. As recently as the COVID-19 pandemic, scientists called for the unit's experimental data on human-pathogen interactions to be released to the international medical community. Both the U.S. and Japanese governments have continued to withhold it.